By my definition the Hilton is already pretty cheap so you may want to give us a price range. I would pretty much agree with others that your best option is just adding an extra day to the Hilton but if you want to save a few extra baht (I cant see you saving more than 1-2000) I would search for some of the hotels on Silom road as it should provide pretty easy access the next day to the Hilton. Well, as easy access as it gets in BKK.

I have stayed here before and it is well priced and clean.

http://www.naraihotel.co.th/