A new gaming rig is looming ahead for me and I'm asking myself that question, just how much am I willing to pay for an upgrade?
For example, the difference in FPS between stock i3 2100 and i5 2500k is around 15% (
http://www.anandtech.com/bench/Product/289?vs=288). The difference in cost is around 130-150 euro (Around $200). Overclocking the i5 broadens the margins by around another 3%, so let's say 20% total difference.
I'm most likely going to take the i5, just because low FPS dips annoy me to the point of desk gnawing. But I'm curious to know your opinion, I consider 20% difference to be worth the extra cash. It'll be a combo gaming and work computer, getting my degree, well one of them this year:P. So while it's a 20% difference in gaming, give or take for margins of error, it'll be a smooth experience for long hours at the desk.
I consider 20% upgrade worth around $200, that's $10 per percent increase, obviously this is not a linear figure:P.