I don't think there's really a formula for this. Charge them what they're willing to pay for it. ;o)
Hey all-
I have a freeware font I created and released out to the web back in 2002. It's had about 20,000 downloads since then. Recently, a person emailed me about using it for a product of 600 units. I have no idea how to price such things or go about allowing as such for this kind of permission. I have used the Search function on this site to help find an answer but didnt have any luck. Can anyone point me in the right direction, or provide an answer to such a request? thanks!