You need something for holding the tanks or does it need to be furniture grade?
If you are on the cheaper side, a bunch of 2x4 and deck screws is all you need. I've build quite a few of these and have done it for a few people on CL. I had one done via lag bolt cause the guy couldn't transport it to his house. He put it together when he took it home.. Kinda like an Ikea kit.
Normally, I suggest making the stand to hold a 75 cause lots of people end up moving up to a 75gal anyway. Best part about 75 is that you can put 4 20gal high instead of a 75gal if you decide to do a 49"x25" stand. The extra deep is good for equipments in the back.
You are looking at most $20 for materials unless you want to use plywood under the tank (which may be necessary if you are not sure the tsmaller ank size you will be using).