If you are not involved in the valuation and/or due dilligence of any of this businesses, then you should only charge a % of the sale. In other words, if you are bringing together buyers and sellers, you could monetize by charging a "referral fee" that usually ranges between 3-7% of the closing price.

It will vary depending if its a B2B or a B2C business model. Usually is you are selling to a costumer, prices are very stratightforward with little room for negotiation. In this case, you would want to first do some market research on your competition and how much they are charging. If your product is very similar to other offerings, you can charge the same, but if is has some kind on differentiation, you should add this to the final price (do some research on EVC formula). In the other hand, if you are selling to a business, usually prices are negotiated depending on the cost savings or increased revenue generated.

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.