For an engine which runs on lower than GeForce 8800 cards, I wouldn't pay anything.
What?!?! A good engine can easily target this level of hardware AND any new cutting edge cards too... its gives your game the greatest market share.
To answer the thread question...
$100 to $200
BUT if it had some really juicy addition, lets say full client server support with a proven track record, Id double the above amount. Also the quality of the tools that come with the engine is a selling point.
To me the most important thing for an engines is speed, speed allows flexibility in game design... it can have the best documentation/samples/tools/fancy FX under the sun but if it can only render a few characters on the screen with uber powerful hardware... then that's a turn off for me personally.