I would definitely just save up and buy heaters. In the meanwhile try keeping the fish in the warmest room in your house, and make sure to keep the windows closed. If it gets cold at night try wrapping a towel or small blanket around the tank to insulate it.
Temperature fluxuations can stress bettas out, so even if its not a balmy 78 a constant 74 is much healthier than the temp going up and down all day.
Oh, and I'm not sure how big your tanks are, but I found a fairly cheap 25 watt heater on ebay, it's adjustable, but I don't know if anyone has any experience using it since I didn't end up buying it.