Cheapest 670 on newegg is 400 USD and the most expensive - 500 USD
Cheapest 680 on newegg is 500 USD and the most expensive - 660 USD
So the price difference is from 25% to 32%.
Unless you are comparing mid range 670 to low range 680 - but that is just stupid.
If you want a fair comparison, look at i.e.:
Gigabyte 670 vs Gigabyte 680 with similar cooling
400 vs 500 - 25%
Asus 670 vs Asus 680 with similar cooling
420 vs 530 - 26%
That's very interesting , and I wonder if you checked the MSI twin frozr , here is below 500 US and believe me in my country things arrive more expensive. Anyway even though been a price tag difference around 20% or maybe a bit higher, still as I told you the guy didn't ask which card would be cheaper , actually all he asked is which one is better. I know most of the time we give advice about the best performance - budget friendly product because I always do the same with AMD (yeah Intel is hella expensive, sorry) but we need to understand there are some people that just don't care if they have to pay more for what they want, and I think this is the case.
My final advise:
If you are looking for a card with a decent price tag and awesome performance, yeah any 670 OC either gigabyte , msi or any other brand will siut you perfectly. But if you don't mind paying a little bit more , the MSI lightning edition of the 680 might be (along with asus directcu2 and gigabyte SOC) the best nvidia single gpu product out there. It will of course overclock higher than a 670 and the extra shader does count on multi monitor and high resolutions. Regards and good luck buying!