Re: David Webb (@Fibbles)
It's not a fantastic dataset but looking at the Humble Bundle stats (because they're readily available), Mac users have been responsible for about 40% more purchases than Linux users. Though Linux have been willing to pay more on average so that's turned into Mac users supplying only about 10% more revenue.
Windows users were only about 63% of the buyers. Assuming that the Humble Bundle and the Steam crowd has a reasonably high degree of crossover (and there's a reasonable case for that given that the Humble guys provide Steam keys as part of the purchase in many cases), it would therefore seem that if you can take what would otherwise be a Windows release and make it a cross-platform release then you can add about 50% to your sales figures.