Glad we can be of some help. One of the easiest cheap things you can do to make your betta happier is make sure he has some cover. If your one gallon didn't come with anything, just get some fish gravel or other suitable rock and put it in in a styrofoam cup on its side. (the rock will weigh down the cup so it stays at the bottom.) This will give the beta a "cave" if he wants it.
Oh, and when you get to that step, I recommend this
filter. You will also need some air line
and an air pump
to go with it, but one pump and one air line is about the same as another, so get whatever is cheapest. Make sure you set the air pump above the level of water in your tank (on a higher shelf or something) unless you want to spring for a check valve
. (Not expensive, but not really necessary either.)
If you are careful, you can probably fishless cycle the filter in your 1 gallon bowl, then add it to your 2.5 with the betta. (Or if you have a 3 to five gallon bucket, cycle the filter in that).
When you get to the heater stage this
isn't great, but it will do what you need it to. Just make sure there is also a thermometer
in the tank so if the heater ever malfunctions you can tell. I normally don't jump at recommending the stick on thermometers, but in a 2.5 gallon every extra centimeter of tank space helps. Actually, a thermometer would be a good thing to by early. Really inexpensive, and will help you know how badly you need a heater. If you can keep the temperature fairly constant with other methods, the heater can be lower on your priority list.
So this whole kit and kaboodle, tank, filter, air hose, air pump, heater may cost around $40, but you don't have to buy it all at once. A betta won't thrive in substandard conditions, but it is very likely to survive.