Take these with a grain of salt. I have medium funded Corsair so I try to price things fairly and according to whether or not I would be willing to pay the same price. 78-79 ATKs are kind of worthless, as anbody who is scrolling is better off getting a Concerto with the same attack. I'd say 8m-9m max. I price 80-83 ATKs at 50m-70m when I sell. I would say a fair price for 85-86 ATKs would be 200m-300m, but I can't be sure since I've yet to see one of these. 87-88 ATKs are worth billions. Personally, I would not buy a clean gun for over 2billion but the auctions are set higher than that. Current auction prices are at 2bil and 4bil respectively. I hope this helps you.
