These are the cheapest trampolines you'll find. If it's under $80, chances are they use wide elastic bands, like an industrial version of the stuff you might find on some waistbands. From the reviews I've read on various models, these are great at first. Bouncy and invigorating like a rebounder should be Then, depending on frequency of use, they seem to degrade after about a year. The elastic bands rip, the mat gets warped and stretched out, and stitching unravels. The warranty on these things tends to be around three months.
There are both high end and low end trampolines that use springs for the bounce. The springs on the low end ones are straight cylinders in shape, while the coils on the high end springs are tapered on both ends. You can see the difference here:
These use a whole bunch of bungee cords instead of springs. Like springs, they come in expensive and inexpensive models. My guess is the price difference is in the materials used. Lower gauge metal frames and thinner bungees bring down the cost and quality of the trampoline. In all the videos I saw where users compared a bunch of different rebounders, they tended to favor the bungees. They offered a softer, quieter, "bouncier" bounce.
Round vs Hexagonal
Rebounders come in two shapes: circle or hexagon. I saw these two types everywhere, yet there was no literature on the benefits of either shape. Why are there two kinds? I found one blog post that helped a little, and it basically said this: The hexagonal rebounder offers a bigger "sweet spot" for jumping springiness. On the other hand, round rebounders have a tendency to send the jumper back to the center if you stray off a little. That's the only difference I could uncover. In the end I decided shape was inconsequential and would not factor into my decision.
I really hate noise, so the noisiness of springs is a big turnoff for me. I don't know if all spring rebounders are noisy, but the high end ones are pretty expensive, so I will probably lean toward the bungees. I'm going to stay away from the elastic bands because I plan on sticking with rebounding for the long term. I don't wan the thing falling apart after only a year. I'll update here once I decide on which one to get.