Most NES prototypes
should set you back less than $100. Think of how many terrible games were released for the NES. The less fun/famous the retail release, the cheaper the prototype (typically) is.
Games like Megaman and Castlevania might set you back $350-$1,500, while games like Championship Bowling would be lucky to bring in $60.
One other factor to consider is whether or not a prominent prototype collector owns that certain prototype or not. For instance:
http://cgi.ebay.com/ws/eBayISAPI.dll...=STRK:MEWAX:IT
This Terra Cresta prototype would have only cost me $33, but instead cost me nearly $200. Why? Because DreamTR doesn't own a Terra Cesta prototype and a bidding war ensued. Had he already owned this game, it very well could have fallen short of the $50 mark. Just some food for thought.