Go to a construction site and as them if they have any wood they are looking to get rid of. You probably need only 2-3 8ft pieces of 2x4 to construct a stand for a 20, then use the $20 to buy half in ply wood to use as a top, and possibly enough for the bottom and sides. Just make sure you are stacking the wood so the weight rest on the wood and not on the screws, this is a big no-no.
Here are some links to a stand I made for a 55gal that I overbuilt. I got rid of the 55gal and have a 80gal tank on it. I in the process of building a stand for a 180gal tank.