The first real question is not a price. It is open-back or resonator, and it is a question about which music you want to make — so answer the style fork before you answer this one.
Open-back
No back on the pot: the sound goes into the room in every direction and into the player's body, which takes the edge off. Lighter, quieter, warmer, and the standard for clawhammer and old-time. If you are playing alone, singing, or sitting in a circle of fiddles, this is almost certainly the banjo you want.
Resonator
A wooden bowl on the back throws the sound forward instead of letting it escape. Louder, brighter, considerably heavier, and the standard for bluegrass, where the banjo has to cut through a band with no amplification. The weight is real — a resonator banjo is a heavy instrument and a strap is not optional.
What actually matters at the low end
In roughly this order: a straight neck, a head you can tighten, a bridge that sits where it should, and geared tuners that hold. Tone rings, wood species, and hardware plating are all real and all secondary — a well-set-up cheap banjo beats a badly-set-up expensive one every time, which is what the setup guide is about.
Buying second-hand
A used banjo is usually the better instrument at any given price, because banjos are bought enthusiastically and played briefly. Check three things and ignore the rest: sight down the neck for a bow, press the head with a thumb to see whether it has any tension left, and turn every tuner to see that it holds. A dull-sounding second-hand banjo with a straight neck is a bargain; a bright one with a twisted neck is firewood with hardware on it.
Bring something you can play, even badly, and play it in the shop or the seller’s front room. A banjo is loud and unforgiving and it will tell you within a minute whether the action is fightable. If you have nothing memorised, run an open G and a C shape from the chord library and listen for buzzing.
One banjo-specific check
Look at the fifth-string tuner and the nut at fret 5. Cheap instruments often put a friction peg there, and a friction peg on the string that never stops ringing is a daily annoyance. A geared fifth-string tuner is worth more than most upgrades of the same price — and if you plan to capo, check whether the neck has spikes fitted or room to fit them.