B Sousa -
Will you have loads of luggage with you? If not, there's a free bus service that serves certain parts of the perimeter of the airport, which puts you within easy walking distance of some hotels.
Here's a map of the local bus service. It also shows hotels served by the "Hotel Hoppa" service.
Heathrow area Bus Map
I've stayed at the Park Inn (formerly Le Meridien) on the north side of the airport -- the free bus stops about 100m from the hotel. It's a nice hotel; the rooms are of good quality, and are reasonably quiet too. At times I've found internet rates for £65-70, although on average they tend to be a bit higher.
Or, pay the £3 Hotel Hoppa fee each way and stay at the Ibis (around £45), Holiday Inn (around £50), or other lower-cost hotels nearby.
Here's another website you might find helpful:
http://www.londontoolkit.com/travel/heathrow_hotels.htm