There is a Travelodge nearby (London Kew Bridge); you can sometimes get a very cheap deal if you book a long way in advance, check the Travelodge website for special offers.
Laterooms.com is generally worth a look for accommodation bargains, although I don't know how good a deal you will get in the Kew/Richmond area, which is generally rather pricey.
Good luck
Mean_genie