The costs of Windows hosting has come down a lot over the years, so it's not the hosting costs that will kill you. More likely, it will be the expense of hiring additional .NET developers. The company I work for runs 200+ sites (most through acquisition), and when we inherit a new site that's not LAMP, it's not long before we migrate it to LAMP, just because it's cheaper to maintain. We have a handful of .NET sites, and a few Ruby sites, and I'd bet that the salary of the 4 or 5 guys who work on those sites is more than the combined salary of the other 50 PHP devs.

That said, if you can get the project profitable enough on your own, maybe you can afford to hire additional .NET devs.