Considering it's method of acquisition was removed when Grey Havens became the Devs of SK, Is it considered a rare Costume Set?
Is the Metal Sonic Set considered rare?
Well, it's rare in that it's old and most of the ones that were bought back then were used quickly and bound but there are far more rare items because the developers enjoy putting things in boxes at 1% and those things bind as well.
The real problem with it isn't how rare it is but how little demand there is for it. Costumes in general are not rare at all since we get a new one every month or so. Therefore the demand for costumes in general is quite low. Metal sonic suit only really matches the plasmatech set as well, which makes it a rather questionable costume choice, and many players are old enough to have bought their own metal sonic suit back in the day if they liked it.
Lego's price guide suggests 5K E for the mask and 8K E for the suit.. which is pretty normal for a med-tier costume type thing. If you're free to play it will at the very least set you up for success trading on the AH or rolling some good UVs for dreams and nightmares, provided you're skilled.
It may not be as common as it once was, but I wouldn't necessarily call it "rare" quite yet. It was given out (free, well ok, you purchase the cd) to several hundreds (if not thousands) of SK players back in the day for getting the sonic CD through steam. Anytime something is handed out in that kind of volume it takes a long time for it to truly be hard to find. This plus the fact that every now and then you can find it on featured in the AH.
Overtime it will continue to get more rare though as costumes can't be unbound once used. I suppose it just depends how many times it shows up in the AH on featured.