Hi Ellen,
Can you say what standard of hotel you are looking for as London has all from the Ritz to the flea pits!!
Also whereabouts in London are you intending to visit ?? Are you looking at the sights of Central London or do you also intend visiting places in Greater London ( or just outside ... e.g. Windsor Castle ).
A daily pass fro the underground only runs to about USD 13 so it may pay to stay outside of Central London and commute in to see the sights.
rgds