That's partially correct, but not necessarily in the way you think.
I've been a pro at several dealerships & independent shops, and I still consult at a few. So I have a lot of experience with a lot of vehicle brands, models, years, & conditions. And I've met a lot of other pros who have even more experience than I do.
So you need to understand that just because you imagine that it's one job you're pricing at 20 shops, it's not. It's 40 or more jobs/prices. Because each mechanic can do a REALLY-thorough, detailed, nitpicking job like he'd do on a vehicle his grandfather used to take him hunting in; or he can do a quick&dirty job for a cheapskate customer who just needs the vehicle fixed enough to unload it on some sucker. That's 2 distinct jobs with WIDELY-varying levels of effort & time. ...for EACH mechanic/shop that prices it.
Then you have to consider the condition of the vehicle... The shop/mech. you take it to doesn't really know how rusty, muddy, patched, welded, or ready-to-fall-apart-at-the-next-butterfly-effect your vehicle is. A vehicle that has either been well-preserved (like in the desert) or thoroughly-restored already (like the one I built for Frank) is much easier to work on, and has much lower risk of incidental damage (like ripping the A-pillar off when trying to remove a hinge bolt), than one that has been abandoned in a closed barn, or hogged through a swamp every weekend for the past decade.
And he doesn't have time to find out the details of its condition, just to give you an estimate - it could take hours of inspecting to get to the meat of whatever particular job you're asking about that day, and you're not going to pay for that time. So he's not going to waste it. He's going to glance at the vehicle & look really hard at you, and then guess the MOST time it might take him, and how much you're willing to pay.
He also has to consider the likelihood of you actually giving him the job, given that you're shopping around. If you're not that interested in getting HIM to do the work, then you might go to 20 other shops - that gives him a 1/20 chance of getting it, so why bother? And why offer you a low price? Just for the privilege of working on your rustbucket? He probably has 3 or 8 rustbuckets of his own at home or behind the shop that he'd rather mess with, if he's not making much money anyway.
And you're right that he probably doesn't have a clue about that particular job on that particular antique - how many eBs do you think he works on each month? Each year? In his lifetime? Do you think he has ever done that job on an eB before? Does he think he'll ever do that job on an eB again? So to cover his @$$, he's going to price it near the high end of what he expects. Or if he's desperate for work, and willing to risk a loss, he'll price it low. But if you try to hold him to that lowball price, do you think you're going to get museum-quality work out of him? Or a happy goodbye?
Of course you have to look out for YOUR bottom line. But to get good work from a pro at a price you can agree on, you have to try to see his point of view, and consider his considerations. No, he doesn't know anything about putting aftermarket A/C in a '69 Bronco. But that doesn't make him "clueless". He can probably identify, diagnose, repair, & road-test an A/C problem in a modern vehicle that you wouldn't even notice in one work day. WHILE getting 4 other vehicles repaired.
But working on antiques is a whole other ballgame, which is why many shops don't even accept that kind of work.
So while you can compile a very-interesting list of prices from hundreds of members from hundreds of jobs in hundreds of cities;, you can't apply any of them to any other vehicle (no matter how identical it seems to you) at the same or any other shop in the same or any other city at the same or any other time.
And you certainly can't "keep the mechanics in line" ;D by telling them you heard on the internet about someone else getting a similar job done on a vehicle you perceive as similar to yours. If someone told me that, I'd just tell him to take his vehicle to THAT mechanic, and see if he gets the same price.
...and my price to him would probably go up 50-200%.