Probably the reason the bolt mounted peep sights are so expensive is that if the sight is removed, there is a rather unsightly groove in the top of the bolt. The alternative to finding a sight is to find a replacement bolt. Good luck with that!
Many years ago ( at least forty) I bought some parts from Numrich, with the thought of converting an 86. I bought a new barrel,a bolt, a set of locking lugs and a deluxe forearm for about forty dollars. I know the barrel was $24.95. After a few years, I gave up on the idea, and sold the parts for about $200. Within a year, I ran across a beautiful deluxe that had been magnaported. Talk about disgusted.