It comes down to supply and demand or "what the market will bear".
I’ve spent considerably more for out of print CDs, particularly Jazz; I don’t consider $28 "expensive" for a hard-to-find title.
It all depends on your budget and how much the music means to you.
Perhaps BGO or a similar label will eventually reissue Ambrosia at modest price. Of course, once reissues are out of print, prices rise again.
It pays to regularly keep an eye on discogs, Ebay and Amazon if you are looking for something rare because every so often a seller will price a cd much lower than other sellers. I've taken advantage of such situations numerous times.