Not a lot. Alfa made a lot of them and owners (like yourself) seem to prefer aftermarket wheels. The condition of the center caps, and of course the wheels themselves, would influence value. My guess would be $50 - $75 for each wheel-cap set if they are in good shape. Shipping costs may be a problem for out-of-area buyers.
Depends on where you’re located and how many other Alfa owners are in your area. It’s all supply and demand. If you’re in the SF Bay Area, you might get $100 at a swap meet. If you’re mid continent, you probably can’t give them away.