I use a desktop lamp (like this
one, for reference) and a 13w spiral CFL that I got from my local home improvement store. Just make sure to look at the lightbulb package and take note of the color it gives. Mine is 6500k. I bought the lamp from goodwill ($2.99) and spent ~$6 on the bulb. I don't think there is an easier way to keep it cheap.
I should note this is on a 3 gallon tank. That might be helpful.