I found a little clip-on light fixture ( I forget what it was called, though ) for like $15 at my LFS. If you can find something like this, it provides some extra 'heating' for a small Betta tank.
I used it for little while for a small 2 gal. before I moved my Betta to a 10 gallon. I turned it on at night ( doesn't give off an excessive amount of light ) and kept the water a decent temp for the fish.