Often wondered the same myself - must be a PA thing, eh?!?!? For what they are, they are AWFULLY expensive! Not much competition, I guess. Never did it yet, but have been interested in taking a long Weaver/Picatinny rail section (similar to the AO/XS set-up), and fabbing up a cheap Weaver-style scope ring base with a ghost ring with interchangeable inserts - even washers drilled to size would work. The bare rail sections are available all over - Brownell's, etc., or can go to a gun shop where they have Weaver bases in stock and try them or measure the base cut-out circumference for one that is close, and go from there. Bedding it with bedding material (Devcon, JB Weld, Steeltex, etc.) would nicely make up for any differences in dimensions where it mates with the barrel Receiver- done for scope bases/rings sometimes. Weaver rings are cheap enuf to start with, but a milling machine would be really helpful here. Redfield used to have a simple peep site that you screwed into the back of one of there bases - simple, cheap, effective and no longer available.
I'm fairly sure the first one would cost WAY more in time than it is worth, but after that, more could be fabbed from a drawing w/ minor changes for each gun. JMHO.