To me, an important part of the equation is what type of riding you will be doing?
These bikes come in so many variations- no suspension, full suspension, suspended seat, etc.
If you will be riding on level flat ground, you won’t really need full suspension, but if you are taking it off road then you will prob need a full suspension bike.
The full suspension bikes have more parts- shocks etc so they could end up costing more than a bike that has a solid frame. If you want to build a drag bike, you will prob need wheelie bars and more powerful engine and brakes which ends up costing more as well.
Some bikes have 4” wheels, 5” wheels, 6” wheels, 10” wheels, 12” wheels, etc there are a lot of different flavors.
So if you want something to bomb around a paved parking lot, you can use a solid frame bike with 4” wheels and a centrifugal clutch and be fine.
But if you are wanting to ride off road up hills and stuff, you might want bigger wheels and a torque converter to help with that.
Starting here on OldMiniBikes is a great place to start, members here are very generous, helpful and knowledgeable.
Knowing what kind of bike you will want to buy can help you narrow it down since there are so many out there at so many different price points, it can be overwhelming.