well it depends what quality of lighting you want most light aren't below $50, i got a t5 set up for about $68 shipped for my 20 high (24in) from
fishneedit. there not the best light but thay are a good bang for you buck place. the wattage the question is do you want to do low light (i recommend under 2 wpg, watts per gallon) or high light (more then 2 wpg) with high light though you will need CO2 because you will get insane algae.
for bulbs for your current hood you could try a hard ware store not sure if thay would have the right length (try it anyway), but most likely you're LFS has the bulb too. ether way bring the bulb with you and make sure the bulb your getting is the the 6,500K to 10,000K range. you can get away with using the curent hood for a long time actually and i did this with pretty good sucsess when i had my first planted tank.
another option is retrofitting the hood you have to use spiral florescent bulbs (just the regular power saving bulbs). i know i've done this on smaller tanks and it has worked out pretty well.
this is one example of what i'm talking about.