Ah my friends, it's not quite that straight forward. By that logic, there'd be no such things as publishers.
I agree $300 would be a bit on the low end but the question is, how much money do you (or any skinner) think they could make selling a stand along skin? Let's say you tried to sell a stand alone skin at $5 apiece. You'd have to sell 60 copies to get to $300, 100 copies to get to $500. That's a lot of copies to sell. I know it doesn't sound like a lot but if you knew the sales stats on most shareware, most of it sells <1000 in its LIFETIME.
So if Microsoft or some other publisher came to you and said, "I'll give you $500 for a skin for us to include in a product we're going to sell..." I'd say that's a pretty good deal.
Pixtudio has done very well but their $8 suites include a LOT of exceptional skins and are full suites, not individual skins.
Now, the other side, $40 for personal use, Joe? Here's a general calculation:
How many hours does it take to make a skin? Then ask yourself how much per hour do you want to charge? Let's say you consider skinning half-hobby half profession and charge $20 per hour and it takes you 25 hours to make the skin. That's $500. That would be a starting point if you want to commission someone to make you a skin. Now if you commission them to make 10 skins maybe you can get a bulk discount.