I used Priceline lately and got the Sheraton Heathrow for only about £55 per night. It got terrible reviews on TripAdvisor.com, but as you can see from my review there (here) I didn't have a bad experience. (There are some good tips on the Sheraton Heathrow TripAdvisor page vis a vis transportation on Bath Road, so it may be worth your while to read all the reviews.)

Which brings me to my pet topic: Hotel Hoppa doesn't visit Terminal 4!! Just be aware of this issue if booking a hotel near Heathrow. (I'm sure there's some explanation, but as I haven't done much UK travel I haven't any details. I can tell you it's possible to make it to Terminal 4 from Terminal 3 in about 25 minutes using HEX, though...)