and the money tradeoff is, how many more FPS does he get with the faster chip?
Well if he gets an x4 940 for example he starts out at 3Ghz with potential to hit 3.6Ghz or higher with a decent cooler, so the performance gain will be significant. I cannot tell you how many frames he will get because that all depends on the game and how his other hardware is configured.
Here is a screenshot back when I had my GTX 260/x4 B50 :
As you can see the combination of having a quad at around 3.6/3.8Ghz gets great results when paired with a 260.
This is what he would get with a stock 940 give or take..