hi
(first i must apologize for poor english)
is it possible to pay unity license price after game sell enough ?
like udk engine don't pay for engine (except 99$) and pay per sell %25.
or something like that?
i ask this question from sales@unity3d but i don't receive answer?
You cannot use the UDK business model unless you've reached some special agreement with UT. And I highly doubt they would give you a special deal though. But there is a Unity Pro trial if you just want to test out the engine for 30 days.
Otherwise you can use Unity Free until you make enough money that you can afford to buy Pro.