How to caluclate a celebrity endorsement equity Deal?

Being apart of the Hollywood scene I've been privy to get some very big named entertainers to endorse my messaging app coming out this summer. When discussing their involvement, they all wanted to take an equity stake in my company for their contribution. I'm happy to offer that up, but how the hell do I calculate the ROI for their tweets and posts on Instagram? Is there a model out there to reference?