The Quadrajet is a very underrated carb. The reason a lot of people don't like them is because they don't know how to tune them. You have to pay attention to the instructions when assembling one after a rebuild so that you set things like the float level, accelerator pump, and choke pull to proper specs. A source of frustration for many is the secondary air valve adjustment. Most people don't know about it, or how to fine tune it. A lot of times, people will disassemble the carb for cleaning, and then they simply don't know how to set the secondary air valve properly.
Prices for used carbs vary, depending on how much the seller values his stuff. I've seen them for as little $20 and as high as $100 at swap meets.