BESX is completely free. My boss was skeptical of this too, but it really is free. 10,000 users, no fees, very well supported (if you have an issue, you can even get 1 free support ticket with RIM), and overall far more robust than the previous non-full-BES RIM offerings.
If you're wondering why RIM would do this, the answer is simple, it's people like you: You have the option between ActiveSync (which is free with Exchange) or RIM's offerings, which previously was 1 free, then pay $100 per new user. But now, you're "free" from such a hard choice as BESX is free too. Makes sense, as it levels the playing field between Windows Mobile phones (ActiveSync integration), Google Android variations (ActiveSync integration), and the iPhones (ActiveSync integration), and now the free backend Blackberry phones. At the end of the day, rather than a cost option, now it is just a choice of personal preference of what kind of smartphone the user/company prefers, not "how much is the upfront costs of each of the phones (well, to be fair, there is still that factor, just to a lesser degree).
For ActiveSync, read up on:
http://technet.microsoft.com/en-us/library/aa995962%28EXCHG.80%29.aspx