Yeah, heat bulbs are really expensive. If he doesn't need super high temperatures it might be smarter to buy a heat pad and thermostat (maybe around $60) instead of buying new bulbs all the time. Either way, it's good that you got that fixed and he's doing well.
It depends, if you have substrate deeper than a few inches and the gecko likes to climb, then it won't work on the bottom. But it's hard to heat enclosures well by putting it on the side or back. So, for the setup you have I think a heat bulb will work best if you're okay with the cost. If you want options, there are also ceramic heat emitters, but you would have to research since I don't know anything about them.
Yeah, a heat pad would be hard with an enclosure like that. Your setup looks pretty good, but a few more climbing opportunities / decorations on the wall would improve it. Maybe gluing a branch/plastic leaves on the background in the middle would help (just be sure nothing will melt from the heat).