I can’t find the eBay listing, and the regal site is a bit tough to navigate. I did find a picture of their top on the site. It looks prettty much like the Prestige top. The description of how it stows and the number of latches doesn’t really match what a stock bird uses.
I had a Prestige top for several years and replaced it with an original late model top. The two are somewhat different in profile (the center bow placement is a little different). The Prestige is simpler and weighs a lot less.
Bottom line is it works, and more or less keeps the rain out. Thunderbirds aren’t exactly leak free with either top in my experience, and I think that’s common.
Either way, there is a lot of fiddling around to get them to properly fit, more so if you use the swing arms to lift them into place. A buddy has a Prestige top on a 57, with the optional front header stainless trim. He made one trip from Seattle to the start of Route 66 in Chicago, to Santa Monica, and back to Seattle. Top up most of the way to use the a/c. So they work at highway speed when properly installed.
There’s a big difference in asking price of originals, but by the time you rebuild one and cover it, it’s going to be a lot more than the Prestige is. I know they work, and the eBay price you quoted is right in the ballpark of theirs. Page 3 of the catalog
http://prestigethunderbird.com