Well, you should consider a few factors:
1. Will you be working WITH the Mail server admin or will the company expect you to perform the necessary mail server admin tasks to set up and configure the BB service?
2. If the answer above is "You do it all." then consider your liability. We all know that a simple installation of one service which integrates with another can be more work than you expect. In other words, they are asking you to stick your hands into a mail server which may or may not be set up correctly. If something goes wrong, what are you in for?
3. Do they want an installation of the BB server and the config of a few handhelds and you are done? OR do they want it set up to work with thier mail system AND design a backup and restore plan for them AND maintain the server (patches etc) AND handle the troubleshooting calls when something, (and it will happen), doesn't work as expected?
I would ask yourself the above question and then outline for this small buisiness what it is you can offer and provide them with options for what you are willing to do. They have to decide what they want and what they are willing to pay for. Whatever you agree on, get it in writing, even if it seems simple now, soemthing may happen and something which effects their business may get lost or broken, and they may come after you. A little piece of paper with an agreement on it signed by both parties can really help.
ok, so how much? well... I would charge no less than $50 an hour... that being said, it is probably more wise to consider something along the lines of a set contract fee for the entire job. Figure 6 - 8hrs to completely set up a machine with BES and test it. Maybe $400. if you get done in 6 hrs. you make more... if you take 10hrs. they don't get overcharged for something you messed up or don't understand.
Also, try talking them into a monthly service fee or something to perform maintenance monthly. Or maybe a $45 per hour on-site support minimum 1 hour per support call.
Just my two cents. Good luck.