Unless you have a business account (or work pays for it), be prepared for the shock of your life.
At this point in time (speaking from current experience with my Treo650), Roger's default data package for any user is 0kb/mb free and $0.05/kb. So to check your email, it could cost you as little as a quarter or a couple of dollars depending on how big you email box is, if you're using pop or imap, etc.
There are packages you have to subscribe to that are laid out here
Telus, Fido (owned by Roger's now) and Bell are probably similar in their fee structures although Fido did have an 'all you can eat' plan for $50/month but I believe it's been discontinued.
It's way cheaper in the states...a friend has a $15/month unlimited data plan...doubtful we'll see anything like that up here anytime soon.