To which cost are you referring? The actual amount you pay shouldn't be affected by this move, as IIS has been bundled with MS' server OSes for quite a while. If you refer to the cost in resources, I wonder if someone who has hands-on experience with Windows Server 2003 could tell us whether kernel-mode HTTP can be turned off?
If so, then there is no cost. Turn it off, use Apache, and there you are. If not, use the Resource Manager to starve http.sys, and use Apache anyway, resulting in (hopefully) little impact.
Of course, you could just run Apache on (Linux|FreeBSD|OpenBSD|UNIX flavor) and be done with it. You'll wind up with better performance of Apache on the POSIX platform, anyway. And, with Linux, you could also use kernel-level HTTP if you really wanted.