selling design rights

I’ve been working with a company over the last year on a freelance basis and I’ve designed a new line of packaging for them. They now wish to 'Purchase Design Rights to Ownership". I’ve been charging them hourly for my work thus far, at a fairly low rate.

Any suggestion as to how I come up with the design ownership rights number? Also, is there any specific terminology I should include, other than being able to show the work in my portfolio?