AMS: The point was to simply illustrate numbers. If you want more realistic figures, keep the Windows price the same as the RISC OS one and count at least 6 years of PCs. Doing 150,000,000 * 6 * [?] * £100.00 = £450,000 shows that you'd need to sell to one in every 200,000 users (0.0005%). And yes, more people use some of my Windows software than there are RISC OS users -- and by a considerable margin.
I develop for RISC OS in my spare time because it's fun to do so. I know full well that I won't make any money out of it, but the years I've spent patching the OS to try and make it dance to a prettier tune have served me well in terms of experience gained.