Donna - try the U.K. version of Travelodge. (travelodge.co.uk)
They often have "saver" room prices that are fantastic for London pricing - more so in advance. I picked up a property of theirs in central London for 19 UK pounds as example.
For the night you are looking, they are showing a property in Croydon (on the direct rail line from Gatwick) for 29 pounds that night. It's outer London, but easily and quickly accessible to both Gatwick and Victoria Station in Central London. Also a heck of a lot cheaper using the local rail, rather than the Gatwick Express.

It's a Travelodge, so no Ritz, but decent & clean enough for a "bed for the night" without the high prices.
Another option is to head south on the direct rail line from Gatwick to Brighton - not as expensive as London, but probably more reasonably priced B&B style options abound.