This is a cool idea, although I'm not really a fan of FDM printed caps. I know
Geistmaschine is working a similar concept called T1, a metal artisan which uses screws to attach different tops.
I'm not sure what your options are using a 3d printer, but I'm curious what your mounting interface is like. This will determine the shape and size of what can be mounted.
Then- are you planning on releasing this information to artisans & DIY-ers for a whole ecosystem of caps, or just GB'ing a different design of yours every once in a while?
Definitely a fun idea but these things are important factors when picking a price. I'd be more inclined to buy this if I can mount things other than assorted helmets.
Personally I would charge <$10 for the base and anywhere from $4 to $50 for tops, depending on designer and material.
I think this has the most potential as downloadable files as well as an in-stock item. It's hard to turn a profit on something this small and cheap without charging an obscene amount of money.