I think the key to remember here is the difference between the "Personal" and "Enterprise" plans. With Cingular, Personal means you're not allowed to connect to a BES, but you can use BIS (personal email). Business means you can use both BIS and BES.
The plans are $34.99 and $44.99 - the first being just unlimited data and email, the second being the same thing but with 1,000 included SMS and MMS messages. If you have a voice plan, they chop $5 off that price, making it $29.99 and $39.99 respectively.
If you want to use it on a BES server, you have to go to one of the more expensive plans. The equivalent plans look like they're $15 more (Enterprise with unlimited data = $49.99... add on 1000 messages = $59.99). These also have that $5 discount if you have a voice plan, making them $44.99 and $54.99 respectively.
If you want work e-mail but don't want to pay more to be on a BES, you can use the desktop redirector, that works fine with the BB Personal plans. I don't know if you'll get in any trouble or not, but I haven't yet.
Here's a link to the table with the prices: Data Connect Table