Just got a Mulong

how much is it worth..? I can only find one in trades with no information...

It's a Donation Item that comes from this month's collection (well this month until tomorrow) So it would be worth probably a little less than the or 250 CSC.

What happens after tomorrow?

Because the May DPs will no longer be buyable from the Cash Shop. It will be June. ;)

Soooooooooooooo if I just keep it, eventually it'll be worth lots?

No, because donation items are rereleased by Luminaire Gifts and Miniature Vending Machines.

Few donation items are worth more than 4-5M

sometimes minions like that go down to 400k