I believe that price zone is any gpu between the HD 7770 and the HD 7870 in price(xt is slightly more expensive) which means if you were looking for modern gpus the gpus in that price zone are:
AMD: 7770, 7790, 7850 1gb, 7850 2gb, 7870
Nvidia: 650 ti, 650ti boost 1gb, 650ti boost 2gb, , 660
If you want my opinion, the 650ti boost(i emphasize boost) 2gb and 7850 2gb offer the most value per $
edit: and on a side note is the gpu to beat as the Xbone uses an apu similar to the 7790 and the PS4 with one near a 7870, you want to have a gpu at least in that tier to play the future games.