I would have thought anything around Heathrow would be expensive as, of course, everyone is looking to park to catch flights.
Your best bet might be to go somewhere west of London, such as Maidenhead and catch the train into somewhere like Hayes & Harlingen where you can change to a train bound for Heathrow. If you are flying out of Heathrow, though, you may not be able to park for multiple days or it might
be very expensive.
Matt