2x4 and plywood tank stand?

[Kabong]

The cost will really be determined by the type of plywood you go for.
I can range in price from $26 as Mark said up to near $100 a sheet.
Remember though you don't need nice board for the back. Or the top in your case.
So you should be able to get away with one sheet and use a pressed wood for the top and back.

Of course another question is do you have the tools to do this?
Or are you gonna have to get them to do all the cutting at the store?