In this case you would need to start figuring out the willigness to pay this facilities have for this service. What is the main benefit of the service? Updated paintworks, worry free maintenance, lower investments?
So, if a hotel is willing to pay $300 a month for the service, you should give back to the painters a proper return on investment depending on the value of the artwork. For example, if a painting has a value of $1.000, I believe its fair to pay them a 10% anual return ($200 year / $16,67 monthly). If the painting is sold you could negotiate a commission of 5-10% and you can also make money on the installation.
Hope this helps you.