What is a fair price? Developers are not cheap and you need to pay many of them every month (or get the equivalent in donated time). We can debate that number of course, so I'm going to start the discussion at $50/year. So your "lifetime sponser" is only worth 3 years (ignoring interest which isn't significant at this time scale).

Accounting for lifetime anything is hard (I don't know how to do the math, I'm sure people that do debate a lot of complex issues), but I'm again going to suggest that a lifetime subscription needs to be 20x the yearly fee to give a number to start the debate at.