Check Craistlist for used tanks. Also, Petco has a weird sale every once in awhile, it's like $1 a gallon sale? So a new 55g is like $55.00, I think they go up to 90g?
Check youtube out for DIY tank vids. I have seen some really nice stuff for pretty cheap. You can use certain Styrofoams and plasters to shape rocks and ledges. There is a special glue and sealant you can use, and it's all readily available at hardware stores.
FYI, seriously tho, anyone who cares about their tanks wouldn't get placo. They get huge (up to 2 feet) and will fuck your tank up. They have to be fed special pellets (they don't live off algae alone.) and shit huge streams of grossness that screw your water up. I had one many years ago, It got to 12 inches in a few months time, and always messed up my plants and rocks as well. Like most aquarists, I hate those things with a passion. But learn on your own.