Whats cheap for you? Whats your budget?
Cheap varies person to person and there are many different options for lights for reef tanks. Honestly, being a reefer myself, i would offer the advice to you that if you really want to do a reef later on, don't skimp on the lights as thats a very important part of a reef tank. Also depends on what you want to keep in your reef. Generally I like enough light to be able to keep anything I want, others want just enough.
You have a few options as far as retrofits/pre-made go. Some cost just as much as the others. Any ideas what you want to keep in a reef? What kinds of corals?
I would
DIY a canopy personally if thats what you really want.