Daox, I know you don't really want to do the expensive LED route, but I have some experience with those...
My friend just recently built his garage. It's 2 cars wide and 3 or 4 cars deep. It's gigantic. He put up 8 led lights (basically one for each car spot) and they are extremely bright. He also only has the framing up, no white drywall or anything of the sort. He got them for about $70 at Lowes. If you go black friday, who knows, they might be on sale. Personally, I would rather get a Lowes product than a cheap eBay, Chinese knockoff. I know you don't want to spend a fortune, but 2 of these I'm guessing would be just fine for you. And I know you're going to do an ROI calculation like you do for everything!
Another thing to remember with the LED replacement mod is you have to find out how wide of an angle of light they produce. If it's too small an angle, they'll be worthless for lighting the whole garage.
I'll try to get a pic up so you can see what they look like installed.