GTX275 is pretty beefy in terms of price, average somewhere at or below $250 or more depending on the features and manufacturer.
I'd say GTX275 competes more with a 4890 than a 4870, however there are likely scenarious where the 275 wins and the 48xx series wins, etc. I'll stop short of calling a 4870 an even match though. It's been a while since I've looked at any benchmarks.
A 4890 is probably still cheaper than a GTX275 though. Just gotta look around and read reviews from reputable places (HardOCP, Tweakbench, Anandtech, and similar themed sites) Then when look at stores you would consider purchasing from, to compare prices and decent which one from which store will be the best deal for your money spent