Watterson is SUCH a recluse these days, I'm amazed he was willing to contribute in any way.
Just TRY to find a semi recent pic of Waterson online. Google him. You'll be amazed how few photos/ videos exist.
I doubt there's a truckload of cash larger than the one they offered him to carry on Calvin And Hobbes. Especially for an independent film that raised just $100K on Kickstarter.
I think the guy who refused millions of dollars through merchandising rights and sales, isn't choosing his projects at this point in his life based on monetary benefits but rather based on the subject matter that appeals to him.