We do it all the time, we generally charge $100/ color. It covers the setup and clean up and amortized cost of the sprayer. It also includes prepping the customer's paint for spraying as it often crusty, chunky and has a paint roller swimming in the can.

There's really not a huge price difference between doing one pair or ten pair so it's a flat rate based on color. So three pair painted to match the living room is $100. Two more pair painted with the bedroom paint color is another $100.

We always use latex paint as it's easier to clean up. This stuff works great to bond to the grills.

[Link: levineautoparts.com]