Depends on the paint your using, I use Vallejo and so I typically use Vallejo thinner. If you're using a lot of it in your process you need to factor in it's expense into your quote

e.g. You bought a $50 bottle of thinners for painting and have 50 jobs to paint, the price you charge should at least cover a percentage of the materials used, in this case $1.10 for thinners.

Also see what you're using the thinners for, is it to just thin the paint or are you cleaning the brushes with it? If your cleaning brushes with it and thinning paints see what cheaper alternatives there are for brush cleaning, have a look on YouTube and see what other hobbyists use.

Good luck