Try the IBIS Barajas. It's only 800m from the airport and you can actually walk to it.
Done this myself last year.
Taxis don't like going there because it is to short a trip. But you can get there by local bus and metro as well with a +- 400m walk at the end.
I now see that from the IBIS site that there is now a shuttle service available to the hotel..
See this link:
[url=http://www.accorhotels.com/accorhotels/fichehotel/pt/ibi/3753/fiche_hotel.shtml]
http://www.accorhotels.com/accorhotels/fichehotel/pt/ibi/3753/fiche_hotel.shtml[/url]
