Usually you don't have much options as to glass or acrylic. Most tanks are glass except the ones that are odd shaped such as wave tanks or bowfront. Glass can't be bent the way acrylic can. Also acrylic is much lighter so a lot of large tanks are acrylic for weight purposes.
As for buying a 30 gallon tank for under $40, you should be able to get a new one for that price. The filter, heater, etc will obviously cost you more but the tank alone shouldn't run more than that. You can actually buy a 10 gallon glass tank at Petsmart for about $10.