A NIB pre-B for $450 - $550 is pretty much the going price. I've owned several, and sold a couple for more than $550.
What's different about a pre-B and B?
The pre-b has a rounded trigger guard. More attractive to most eyes.
The pre-B doesn't have a firing pin block.
The pre-B generally has a much better trigger, but there are exceptions.
The pre-B has a spur hammer, which can bite some hands.
The NIB pre-B doesn't have a warranty.
The pre-Bs have different sights than the modern versions, and finding replacement sights can be difficult. (They use a narrower dovetail on the rear sight, and the front sight is staked, with no dovetail.)
There are a few parts that are gettng rare for the older pre-B, (from the early 80's) most associated with the safety. Those parts almost NEVER break or wear out.
Many parts are compatible with the current gun, but a few are different (including hammer widths, depending on how old, etc.)
Some pre-B are "transitionals" with squared trigger guards and rounded rather than spur hammers. They look just like the current 75B, but don't have the firing pin block.
The current 75B with the firing pin block is harder to tune for a good SA trigger, but a good gunsmith with dies can do it.
Are the pre-B's worth the price? Beauty is in the eyes of the beholder. I'd rather buy a NIB 75B and pay to have the trigger improved by a good CZ gunsmith. Or I'd buy an 85 Combat (which is pretty much a pre-B with a warranty.) But some folks REALLY LIKE the pre-Bs and are willing to pay for them. To each his own.