Ouch! It's something I need to look into as well as need to get mine changed. I've been with Specsavers for a while and they aren't bad on prices and I get a discount as I'm on their direct debit for contacts.
I paid extra for a thinner lens and anti scratch last time though can't say the anti scratch has made much difference, if anything I've never had glasses which have got so scratched. Will also reconsider the thinner lens, as my prescription is higher the lenses still stick out a bit at the back anyway as I like thinner rimmed glasses.
I went looking with a friend recently for hers and she tried some independant places which seemed a lot cheaper and places like the larger Asdas seem to do them as well. Definitely going to shop around so will be interested in hearing where you find the best value.