The cheapest imports new are ~ $250. Have you checked eBay? Some good bargains to be had.
If you really want to build one, that's cool but it will be a challenge and ultimately it will still cost a bit for materials and such.
Simple dividing heads can be made with nothing more than a spindle and a gear on the rear with a finger/plunger which allows indexing via the gear itself. It'll need a spindle lock to prevent movement during cuts.
For example, if you have a 64 tooth gear on the back of the head, you can do any number of divisions within 64, for example 64, 8, 32, etc (it's early and I can't do math). Swap the gear out for other divisions. It won't have super accuracy but might work for a number of applications.
I think ultimately an eBay buy would get you going quickest. Gear form cutters aren't cheap either, unfortunately.