There are several factors in the price of a Generator.
The quality of the gas or Diesel motor, the size of the motor, the type of generator (is it an inverter or a conventional), and the intended purpose for the generator.
The more established gas motor manufacturers make different quality motors. For instance Briggs and Stratton has several different quality levels of motors. They have different series of motors. Sprint, Intec, and Vanguard are some of their series of motors.
A camping generator that gets 20 hours of use per year does not need to be as durable as a Contractor's work generator that will get used 20 hours a week.
You can get a 6,000 watt generator for as little as $450 or you can easily pay $1700 for a tri fuel commercial grade 6,000 watt genset. While they will produce about the same output the durability and efficiency of the units will be significantly different.
I have an affinity to buying good stuff and keeping it working. I am a big fan of both the Yamaha and Honda generators. Their Inverter technology is quite efficient and durable. Also with many dealers the availability of repair and replacement parts is excellent.