Its looking likely GTS is going to be 'around' £210 retail.
Also its not exactly easy to gove a direct comparison between GTX and new GTS as they did not do a direct back2back test, but if you look at the Prey 1600x1200 results on these pages:
GTX @ 112FPS (http://www.tweaktown.com/reviews/1129/8/page_8_benchmarks_prey/index.html)
GTS @ 155FPS (http://www.tweaktown.com/articles/1244/7/page_7_benchmarks_prey/index.html)
GT @ 143FPS (http://www.tweaktown.com/reviews/1210/7/page_7_benchmarks_prey/index.html)
Please understand that these are not directly comparable, but should give some indication of where the performance will sit if you can allow for hardware differences.
Direct comparison in Prey.