Looking at multiple examples: The average fee that I see is about ~2kg per accent, or 3kg per skin (prices may vary) plus blueprint costs for the commissioner.

Higher end artists, such as Spikulec, whose slots sold for $75 or ~7500g apiece, will probably charge more.

So depending on who you're asking and your coverage, I would estimate a minimum of 6k gems, ranging up to maybe 15kg if you're really desperate and it's a popular artist.