Free is speech, not beer. We do have to be fair to Windows, so let's overengineer a ROFLscale Asterisk PBX for comparison's sake.
First, a Digium-supported Asterisk installation at $11k for a three-year term. Since that's a five-server support agreement, we'll make this a three-system cluster just because we can. And since we're doing the support-contract route, that's three entitlements for RHEL at $800/ea/year ($7200 for three years). Buy the hardware; tart up some R720s with crazy RAM and call that $6k each. Add a nice switch (heck, add two!), firewall, and we're still at $35kish. I'll call that a draw based on the infamous parity exchange-rate concept; if you disagree, throw in training for your PBX guy and some one-on-one time with Digium's people until the prices balance out.
Oh, wait a second... we didn't buy Windows. That'll cost you $2k for those three machines (Windows Server 2012 Standard). Whoops, now the Windows software alone is more expensive than the Linux hardware and software.
My coat? It's an OpenBSD fireman's jacket.