There are LOTS of Q-jet sources. Some will get you a good Q-jet. Some will get you a pile of mis-matched parts, & a waste of your money.
Don't think you'll find many GOOD, CORRECTLY rebuilt Q-jets, for less than $400, today. I'm sure there are a few exceptions.
Most Q-jets that originally came on some popular Musclecar will cost more. If the car was a rare, high performance model, the Q-jet could cost in the thousands.
But, a correctly built Q-jet that came on a "Plain Jane" car, or most any non-Musclecar, will usually be cheaper.
There are several different choices in the model of Q-jet you choose. There are front inlet & side inlet models. Some front inlet models came on Pontiac, Olds, Buick, & even Chevy vehicles, & maybe other brands as well.
There are side inlet models that came on GM vehicles, as well as some sold by Edelbrock, in the 1990's thru the early 2000's. As far as I've been able to determine, neither side inlet nor front inlet style has a performance advantage over the other type. So, the type of inlet is just your choice.
If you choose a front inlet model, you can choose one that came on a Pontiac, with a Pontiac style throttle lever. The '70 & earlier non-CA Pontiac models had the smaller bowl vents. '70 CA models, & most '71-'79 Pontiac models had the big "smokestack" vent, which requires a hole for it, in the air cleaner base. '75 year models are to be avoided. '76-'79 models are desirable for performance rebuilds, because they have the APT feature & because they are 800 cfm.
Early '70's front inlet Buick models are desirable, because many are 800cfm. The Buick & Olds throttle levers are different than Pontiac, therefore will require throttle linkage mods. Later model Pontiac models also have different throttle levers than the early Pontiac Q-jets, such as those that came on 1st gen Birds.
750cfm models are plenty big for most street Pontiac engines, including 455's. Most all '70-'74 D-port 455 engines came with a 750cfm Q-jet. I used 750 Q-jets on all my 455 bracket cars, back in the '70's & '80's. Plenty of carb, down into the 11 sec zone. So, they should be good for most street 455's, unless it's a big power engine, such as 500hp +.
Here are just 3 more Q-jet builders. Lots of people buy parts from Cliff Ruggles, and rebuild their own.
https://www.smicarburetor.com/products/sfID1/28/sfID2/9/sfID3/100https://quadrajetpower.com/http://www.everyday-performance.com/quadrajet_carburetors.htm