I agree...glasscages can do almost anything, and a lot of people on monster fish keepers have used them and are quite happy with product and price.
I would look at a pond instead...really, a snapper should have more like 6'-8' x 4'-5' and 3' tall, if you don't want him letting himself out....and even a 6'x4'x3' tank is going to be pricey, very heavy, and difficult to move. You can do a lightweight indoor pond of that sort of dimensions for under $500...and it will be pretty much unbreakable.
Yeah, join a fish keeping forum and you're bound to find good deals here and there.
I'm going to build a plywood and glass aquarium. A guy called urajoey or something on youtube uses liquid rubber. which is apparently safe for fish. If it's safe for fish it's definitely safe for turtles. fish are more prone to water impurities than a turtle is. Guy used it for a tank that now holds 2 stingrays. Stingrays are amongst one of the most sensitive fish to own. he's had em for 3years in that tank. So I can work out those dimensions no problemo. Good thing is it won't be too hard to dismount if I need to move eventually. I don't think they need 4feet wide. That's overkill. 2-3 feet wide and 6-7feet long is good enough. Snappers "can" be active. but they're not like a slider. So they're not going to be running up and down. Which would be different. If a slider got that big it would need twice as big as the snappers. It's going to be a common snapper.