In his build, the FX-6100 was $105 and the fx-6300 was $140. That's $35 an 33% difference. On a budget build, that's a big deal. I agree the fx-6300 is a little bit better, but not 33%.
And no, I wouldn't pay 33% more for a i5-3570 over an i5-2500, because they perform about 5% different. Same logic.
You can google the benchmarks, but if we are talking about a gaming rig, I think most would agree an i3 is going to be a better buy.