if you have an overnight at Heathrow, some of the cheapest hotel options are along Bath Road on the north side of the airport — and there are tons. you will also hear about the Hotel Hoppa bus service which will take you door to door (well, along a route) for £4 each way.
that’s over $13 round trip! HEYELLL NO.
i learned about the Heathrow free (yes, free as in beer) bus system via some posts on TripAdvisor — and it’s true! there is a zone around the airport called the Freeflow zone and all the buses that have lines through it are free in this area.
the ones of interest run from the Central Bus Station, easily accessible from Terminals 1, 2, and 3, and reachable by shuttle train from Terminals 4 and 5 (follow the signs to the bus station and/or T3). the map that’s linked from the Heathrow website is slightly out of date as of today in that there are no more bus stops 1 to 5. the ones i needed (75/76/441) now run from stops 19-20. the map lists several of the larger hotels but you may want to just look yours up on Google Maps to see if it’s near any of those in case the hotel itself can’t direct you.
there is an information desk at the Central Bus Station — ask there if you’re not sure of bus routes and double-check with the bus driver that they will indeed be passing by your stop. you won’t get door-to-door service, but my hotel (the Arora) is pretty much right off one of them.
unfortunately i’m not handy enough with the Transport for London website to tell when the buses stop for the night, but you can use it (although it’s not the most straightforward with some of the addresses) to plan your route. depending on how you look it up it may give you different results, so moral of the story, find a human to help you out if you’re not sure and you’ll save some valuable money!