They used to be $5 each, after the Model 3 came out, they increased the price to $15 each. It takes a while to get them though.
FYI, all Tesla S X and 3 caps are the same size. The gray Model 3 center cap set on Tesla’s website is the best deal at $50/set.
If interested, I have 4 T-Sportline caps I will sell for $20 including priority shipping to anywhere in the US.