The Bounty I started is up to $475, with little publicity and folks having to trust a stranger with their money - so that should tell you something.
Mac users are definitely a small market share, but they tend to be much more "active" with their computers and are willing to pay for good 3rd party software, etc. which explains the lively small developer community.
My take: you'll be selling to a smaller market, but you'll have an opportunity to sell to a larger segment of that market.
With a $50 pricetag and some good press on the Mac sites, I think you'd get a lot of takers.
I sure would love to send you that bounty money!