Depending on the size you want it can be quite expensive for just the materials. Say if your looking at a 200 gallon tank you would likely want 3/8" tempered glass. You would want to reinforce the corners so they don't pull apart and use silicone to seal against the water. Make sure you get fish safe silicone as some types will kill the fish. If your looking for a 50- gallon tank you could get away with 1/4" tempered glass. 1/4" tempered isn't all that expensive. The trick is getting a good seal so it doesn't leak. Glass will go up in price if you get the edges polished to make them pretty, but if you put a wood frame around the edge you wouldn't need it.