Expense comes from features.
A simple POP3 or IMAP mail store with no spam filtering, no backups, no redundancy and no support is cheap to set up, and can be sold cheaply.
An enterprise-grade Exchange environment with extensive spam filtering, nightly backups, compliance archiving, fault-tolerance/redundancy and excellent support by competent technicians is hugely expensive to build and maintain, and comes at a commensurately higher cost to the end user.
If you need a "middle ground" I believe Google still offers a commercial version of gmail as part of Google Apps for Business. I'm not sure if it's within your budget, but it's a great option much closer to the second scenario than the first and it comes at a not-too-horrifying price point...