The amount isn't too far off I don't think. I'm no font designer, but four man years (including overhead, etc...) isn't insane, nor is that figure off for expected salary.

That said, I'm very mixed on this kind of "ransomware" development. If you want to share, share. If you don't, don't. But front-loading the costs of your proprietary software product on people who actually want free software is borderline unethical, IMHO. Obviously people are free to pay if they want to, but if you really want to support free development there are better targets.