Well first of all, we're guna need the theme you want to work with (or at least your personal color).

Personally I'm not a fan of a costume being a pre-existing set that you drench in the biggest, shiniest, most expensive accessories. If I had a dollar for every "costume" that is just a jazzed up Chaos Set or an accessory encrusted Black Kat set, I'd have enough money to make every Kat Raiment in the game. It can look nice, but a lot of the time it's just sorta... "Look at my money!"

Personally, I'm a Military themed myself, so these are the sort of things I normally work with...
- Fractured Mask of Seerus is a nice and relatively cheap costume helm. I don't see it around that much, but it's somewhat accessible. I like how it looks with Horns or Mohawks.
- Ash Tail Coat is easily my favorite costume armor. Admittedly, it's closer to Heavy than Military, but it works with a lot of Military themed helms. What's nice about Wolver Coats in general is that various upgrades fit with a lot of themes. 2* Wolver works with Regal, 3* is Toasty, 4* is Heavy, Skolver is Divine, Vog is Volcanic, Snarbs is... shadow?
- Elemental Hood, similar to Wolver coat, is a simple hoodie that fits with a lot of themes. 2* is... arguably Cool? 3* is Military, 4* doesn't fit too well with stuff, Chaos is Dusky, Divine Veil is... well... Divine, and Grey Feather is Toasty. The Grey Feather Cowl gets a special mention because it has built in Glow Eyes, which is normally a VERY rare/expensive accessory. The Cloak works too, but I find them more meh, compared to other armor options. Something about the silhouette is a bit off when you have another helmet.
- Dread Skelly Helm is a fun spooky alternative helm. Similar to Ash Tail, it's closer to Dusky rather than Military, but I find it works well with just about anything gray. It also has the built in glow-eyes, which is another fun plus. I guess the armor works, but they're so... weird. It's clear the set is meant to mesh together, as the rigid teeth on the helm match with the lower jaw on the armor for a weird spooky neck... mouth... thingie.
- Bomber Helms are a very underrated "blank slate" type helmet. If you got plenty of helmet accessories, you can dress up Bomber Helms however you want. Like Wolver/Magic sets, each upgrade has a different theme. 2* is Regal, 3* is Military, 4* is Heavy, Volcanic Demo is... well... Volcanic, Mad Bomber is sorta it's own thing, but kinda works with Toasty and Heavy (BTW, it has a built in prismatic mohawk which is sweet), and Bombastic Demo is Dusky (And Mercurial Demo is it's own thing). Personally, I like them with Mohawks (spike or otherwise), but they work with pretty much every accessory due to how much of a blank slate they are.
- Nameless Poncho, to my knowledge, is the only prismatic themed armor. It can be a bit odd to thematically fit into something, due to the cowboy/native american styling, with prismatic spurs and six-shooter wrist... things? The other Gunslinger Ponchos can work I guess. Personally I find the 5* ones to be a bit too... fancy and have their own unique stylish elements that kinda sorta somewhat works with/against a lot of other things (namely the 6-shooter wrists and the spurs)... Although Deadshot has a smexy skull on it's back. The Regal Sunset Duster and the Cool Gunslinger Sash are neat.
- Dragon Scale Helm and the Wyvern Scale Helm are both rather neat looking helmets that aren't that common. Dragon is Toasty, Wyvern is Cool.

Special mention to the Starlit Demo set, which have unique models, but are a super weird color scheme that is nearly impossible to match with anything other than super rare Turquoise stuff. Same goes with the Starlit Hunting set, so you can try matching those two together for something that most people forget even exist. If you can get them to work, they are VERY cool looking unique models.

Wow this post ended up being longer than I expect. It's mostly fueled by my dislike for the "Look at me I'm rich!" costumes that I see all the time. So please don't be one of those diamond farting Black Kats (Unless it's purple diamonds... because... well... Black Kat set is purple).

A few costume ideas:

Starlit Demo Helm/Mercurial Demo Helm/Kat Hiss Cowl with goggles look so hilarious.

Kat Claw Cowl with Dusker Coat. You can add a mohawk and round shades to look extra cool.

The winter viking armor and azure (blue) bombhead look good together. The emerald bombhead looks great with mercurial demo suit as well.

Stellar sentinel helm and pullovers look good together.

Ironmight plate shield can work as a costume shield in haven.

You can add glasses and a toupee to a winged helm. They give you kind of anime main character look. This is one of my favorite costumes. I have a volcanic winged helm with frosty glasses and toupee paired with volcanic spiraltail mail with ghostly aura and dusky bunny tail. I was gonna add a volcanic tome of rage to it, it would've looked even better but it was too expensive for me.

My favorite costume of all time is stellar sentinel helm with grey mohawk and tech blue closed jacket with tech blue canteen. Makes me look like a blue version of the alien character in loony tunes.

Personally I like accessorizing my main set since I play LD a lot costumes are only for Haven and occasional vana/D&N runs. My current costume is bkc with dusky plume, spike mohawk and glasses with the mail having ghostly aura dusky cat tail canteen (will remove it soon cause I'm getting bored of it) and dusky side blade. I'm currently searching for dusky valkyrie wings to put on it and make it my main costume.

I used to have a bkc with dusky long feather wide vee and knight vision goggles and it looked so bad it was hilarious. A very bad costume makes a good costume, don't forget that.

So personal color is subjective here since with style kit after you acquire one you can change anytime for 1kcr.

You want a black theme costume... Well for your budget is a bit hard to find unless greyish color things are ok.

I searched up a bit myself for black stuff, and you could either use Iron Wolf/Dragon pieces, Snarbolax is too mainstream and i won't even mention it since it's a bit more for dark blue than black, Fractured Mask of Seerus is around black-grey color scheme so it should be fine to either pair one of those, Firebreak Helm (pig tails one 2*) has a nice shade of black apart from the prismatic plumes and the orange chin part that you can easily hide with a scarf.
Also Black Fowl Cloak could work with either the same Black Fowl Cowl or a bomber helm the 4* one to be more precise, or you can also use Rock Jelly Helm, it's very versatile you just need to know what do you wanna place, toupees are very popular on those kind of helm to give you the "sir" look xD.

For acessories I recommend either Cool (more the ones towards grey rather than blue), Grey, Heavy (more towards grey and brown), Polar and last Shadow (these depending on the type are a bit overbudget if you look for certain pieces such as scarf or vents)

GL on your search for the "perfect costume" :)

First of all while you figure it out ask yourself this, what projects a part of yourself?

Reach into your personality; pick the set that feels right you you. And then add the right accessories to it that compliments/amplifies.
Choice is an extension of oneself, so you will need to do most of the work and search yourself.

The best solution is certainly by experimenting in SpiralSpy once you learn how to use it. Good luck.