I think most people overlook the true costs of an Exchange installation, and I see that going on here as well.
Even if you go with the $4200 set of Exchange licenses, you haven't factored in messaging antivirus, antispam, and a backup solution. Barracuda makes some excellent appliances for antivirus/antispam, but you'd be looking to spend $4,000-$6,000 plus $2,000 to $3,000 per year in service renewals. Backup is also not cheap for Exchange, as you will need something that has an Exchange agent.
Now, you're $4,000 project has turned into $10,000 easily with 30% yearly recurring fees.
In almost all cases a hosted Exchange solution is going to be cheaper. I have one small customer on Liveoffice.com and they have 3 1GB Exchange mailboxes for $9.00 a month. Another customer just signed up with TimeWarner's hosted Exchange service and that runs $7/month per user. If I had to recommend anything right now, I'd recommend TimeWarner's service for sure. It's been flawless. Twenty users should cost around $140/month, and the time you'll save not having to mess with the server is priceless.