Nail art pricing - help

Hi everyone,
Am new here and just starting out as a nail tech.
I’ve worked out my price list but am struggling with the nail art aspect.
My biggest question is:
How do you charge for Swarovski Crystals?
Do you charge by size? The number of stones?
If they’re standard or AB stones?
I honestly do not know how to charge for them.
Hoping for some advice (GBP currency)
Thanks xx