There was no cost to you in either case. You made money ;)

1) You made 359991cr
2) You made 9810cr

As far as I know, it IS a flat 10% off the sale price.
However, the posting fee is included on the return, so maybe that is confusing the OP?...

If the items sold, the cost is a flat 10% of the sale price. If the item is not sold, then the cost is the listing fee, which is tricky to calculate. The first reply to your post gave the correct answer. The amount you get back in the AH mail is different since it is both refunding the listing fee and deducting the selling fee.

Edit: I guess there are also a large number of opportunity costs and risks, such as time spent listing, monitoring, dealing with mail, etc. and risks of mistyping numbers, putting the wrong thing up for sale, etc. Those can end up being quite expensive, but the costs can't be calculated based on what the OP provided and the OP says that they provided all the information you need.

Let me guess, are you factoring outside things like CE-Crown conversion in these costs?

The givens in the beginning don't suggest that though.

guyjin... I don't think you even understand your own claim. If you add up all the costs, you did not just lost 90 crowns, if we go by it being a 10k recipe purchase from Basil.

10,900 - 10,000 - 10,900(.1) = 190

The problem is your initial situation wasn't even mentioned that it was just a recipe in the first place. That is not to be assumed automatically, and the multiple factors behind selling an item for the 399,990 (which I assume is a 5* in AH, but I would never personally make that sale in the first place since the margin is much lower than I'm used to) is too much to account for (mats if you had to purchase, recipes if you had to purchase, opportunity costs, etc.)

The only cost in selling an item in the AH is the 10% fee on the sale price (or listing fee in event of non-sale)

For example, you create an auction for a wolver cap at 3K buyout and 2 days auction time. The listing fee here is 300 crowns which is immediately taken. If the item gets sold, lets say at buyout of 3K, the listing fee of 300 crowns which you had paid earlier is returned to you. Also, a fee of 10% of the sale price is taken from you, amounting to 300 crowns.

So the cost is 300cr(original listing fee) - 300cr (refunded listing fee) +300cr(10% sales fee) = 300cr

Okay, so the second sale was of a 4* recipe. Like Volebamus said, there's no way anyone would automatically assume and decide that it was a recipe that you sold. But that's only a minor problem.

This is just a repetition of Volebamus's calculations:

-10,000 CR for buying the recipe off him (-10,000 net gain)*
-1,000 CR for the listing fee, as you stated (-11,000 net gain)
+10,900 CR for the final sales price (-100 net gain)*
-1,090 CR for successfully selling the recipe; 10% sellers fee charged (-1,190 net gain)
+1,000 CR for successfully selling the recipe; listing fee refunded (-190 net gain)

However, this is only the net gain/loss from selling the recipe, not the cost of selling an item. Yet even if you find that by taking away the starred calculations above, the final answer would still be the 10% sellers fee (-1,090 net gain) that you're arguing against. The problem is that, per response #10, you did not factor in the listing fee that you put in for selling the recipe. That makes your argument look like you assumed that you received that listing fee refund for free.

Unless we're still missing something from what you've said, your argument that "the cost of selling an item is not the 10% sellers fee" is the misconception here.

Finally, when you put an item on the auction house you must consider how much it cost to you, divide that cost by 0.9, that's the minimum price you have to sell it for to break even, assuming you manage to sell it. If you sell at any lower you are losing money, if you don't sell you are losing money.

Most people don't understand this that's why you see many 4* recipes for less than 11112 crowns on the ah. They believe they are just recovering their initial expenses. Such apparently-hard-to-sell recipes would be better sold on trade chat if you don't want to take a hit on your finances. They are hard to sell so there's a big chance they won't even at that price. And the more you lower to get it sold the more you lose for having sold it below price.

If you hold on selling the item, even if you don't try the trade chat, then you give a chance for those items already on the market to be sold and then you can put yours up instead of increasing the competition for who can sell first, and lose more at that.

xd my programm can calculate the auction house prize...
http://forums.spiralknights.com/en/node/18094

Yeah I was about to say something here along the lines of guyjin not really understanding the whole math thing, but it seems he got it. Gotta check them numbers!