For under $50? Plexiglass sheets are expensive for small pieces ($13 for a 12x20 inch piece). The aquarium silicone is $14 a tube around here. You may be able to use GE Silicone I - that's about $5 a tube. I'm not sure how much a large piece of glass runs, but I've heard that it can be expensive as well.
Honest though, I think you're better off checking the local trade websites like kijiji.com, craigslist.com, and freecycle.com. Yard and trunk sales, too! You should be able to score a 55 gallon easy for that. Perhaps even a 90 gallon if you get lucky.
And right now Petco is holding a $1 a gallon sale. So you could buy a brand new 40 gallon (breeder tanks are recommended) for $40.
