2-23w 5000K'S= $19.99each
or
2-13w 5000-6500K's= $11-$13 each
2 Sockets and reflector $12-$15 each (or some other cheap set-up)
Maybe later on a 20W Actinic tube or two + Ballast $25-$50.
For a 20G this is more than fine. I use the 2-23w 5000Ks(with a good reflector) and a single 20w Actinic tube. The tank is nice and bright. I have Monticaps, Digitata, LPS ,Softys, Polyps, Mushrooms and a Tube Anemone. All are doing more than great

. People always ask me what kind of lighting is on it, and they are always impressed.