Prices seem outlandish for these things, considering how little there is to them. I am planning on modifying a generic one: basically 3 bows and a distance between uprights of roughly 80 to 82 inches. Stainless fittings are readily available, on Amazon and elsewhere. Trickiest part will be the backstay hole, but that looks lame even on the $4000 biminis. I have other things I would spend four grand on first.
The cost is mostly in the labor; not the materials. Plus, there is overhead of owning a shop, etc. I used a generic, premade Bimini on my Pearson 30 for a few years. White only available. Installed it myself but it did not go past the backstay. No modifications. Back then in 1993-‘95 it cost a couple of hundred dollars. When the first one started to look shabby, I just got another one to replace. Eventually, however, I did have a custom one made.