My wife has a blackberry for work and she has most of the packages that come with it which are email, internet and regular phone stuff, which runs $160/month - more then all the other cell phones in the house combined. The AT&T bill I have lists all the blackberry charges with blackberry specific titles, however since we have a family plan I'm unable to tell if that would exceed a non-blackberry plan. You'll probably want to contact them to compare costs with the features you're looking for.
Personally I hate her having the Blackberry, it has an extremely annoying light that flashes constantly for some unknown reason and she spends a lot of time on it in the car which used to be time we had for conversation which has now become time for email. The irony is that I dislike it because it's performs the duty it's designed for so well.
Options-->Screen Keyboard --> Led Coverage Indicator
(I have a blackberry 8700, hope it's the same.)
If it's red, check your mail.
I can't comment on value as mine is provided by work.