Please check the Dungeon Defenders trading threads in the MMORPG General forums:
http://forums.d2jsp.org/forum.php?f=216 before trading/buying mana here. I've noticed that the mana selling ratios in several threads here are not up to date and you will be ripped off if you buy at those prices.
Current mana selling ratio seems to be 20 mil mana vs 100fg, although there is one seller who is doing 30 mil vs 100fg. Prices will most likely continue to drop. Do yourself a favor and don't let yourself get ripped off.