Anything in the $100-$300 range is going to be low quality chinese.
Imo, anything with a single belt drive is going to be low powered, and have poor resolution.
Something like this is the best option, imo.
4th Axis with Harmonic Drive – Build Log | Carpenterswoodworking.com
Expect to pay between $200-$400 for a quality gearbox, unless you find a really good deal. I picked up some zero backlash 21:1 gearboxes for $250 each on Ebay, but they usually go for $400-$450.
Here's another good one.