This is a classic case of the dealer quoting you the factory price for the factory part. Which means the part will be brand-new and identical to the one that’s broken. The problem with parts like these is that they don’t tend to break very often, so there are no cheaper brands of replacement parts available.
The first thing to do would be to try an independent workshop, maybe even a Mazda specialist, who is very likely to be able to carry out this job for considerably less money (and I agree; $2000-plus to replace a manual seat base in a five-year-old car is getting up there).
Your second option is to do what older-car owners having been doing for decades and hit the wrecking yards. You’re looking for a crashed CX-9 which an undamaged seat base. Which, if it’s the same model and trim level, should bolt right in with few hassles. Make sure the replacement base you’ve found hasn’t been bent, twisted or otherwise damaged in the crash that led the car to the wrecking yard in the first place. But all things being equal, it would be a simple job for a competent mechanic and could save you many hundreds of dollars.
To be honest, like you, I’m a bit surprised at the quoted price for a new seat base, given that the adjuster is a manual one and not electrically-powered, which should mean it’s a pretty simple device.