What are your tips on purchasing a gravity bong?

When I was younger, we would make them out of a 2-liter bottle and a kitchen sink. That was fun and all, but a PIA and I'm adult now and I don't want to recreate that experience.

I'd like to buy a gravity bong, but I'm not sure what to look for. I see the Studenglass looks damn sexy, but I don't have $400+ to spend on such things. There has to be a good middle between $1 bottle of pop and the $400 rig.

What characteristics do I look for? I've bought piles of vapes, pipes, bongs and other devices that collect dust because they are not right. I'd like to avoid buying a bunch of these to find the right one. Any advice would be appreciated.