Sorry for jumping in like this as a newbie on this forum, but I think some of what has been said here requires a comment or two.
First, there's a lot more to running a communication system than just software investment cost. In addition, Exchange is not just popular because a lot of business owners have heard about it, it's popular because it excels in virtually every IT area contributing to TCO, compared to other products (scalability, security, reliability, administration, integration, usability, accessibility). Can Exchange get pricey in larger deployments? Absolutly. Is Exchange the right tool for "just a couple of POP mailboxes"? Absolutly not. Bottom line is, you get what you pay for. This has nothing to do with percieved views of products or vendors, this is a well established product (more than 10 years) that has an extremly large infrastructure surrounding it. That's why business owners choose it - if there was something with a lower TCO that would supply the same features (by that I mean all the above mentioned), I am convinced businesses would go for it (example: the shift from Domino to Exchange - Domino used to be the market leader).
Aside from that, the startup costs for an Exchange Hoster (Software only) are around $60 (ca. $30 each for a Windows and an Exchange CD through SPLA). The rest is billed on a monthly usage basis.
Rgds,
Marcus.