@notfitkid Wow, ok I have no experience whatsoever for this PR stuff either. I think it's fair to just be honest and candid with the client.
If I were you I'd just say "I have to admit I've never been asked to do PR events before as part of an art contract and I'm a little stumped as to how much to charge for it. Could you help me out and let me know your budget for this project? It would help me a lot to get this moving to the next stage! I could be here all month trying to come up with a quote otherwise haha.. Thank you!"
Although to be sure, this offer should be a boat load of money. It includes rights to usage forever (exclusive of no?), lots of promo and lots of your time. If they offer something like $2,000... F off!