Blackberry data plans always cost extra, but they are usually tiered in BIS (Blackberry Internet Service - cheaper) and BES (Blackberry Enterprise Service - more expensive). You would only need BES if you worked for a company that supported Blackberrys. As to why they cost more, I'm not sure if its a licensing thing from Blackberry (because your email runs through a private Blackberry network) or because its perceived that Blackberry users will use more data (which is generally not true).

As far as editing documents on the Blackberry: it will be more difficult because the BB doesnt have a touch screen so navigation may be difficult. I also didnt like the implementation of Documents to Go on the BB, but I never felt comfortable editing documents on any device that wasnt Windows Mobile (and I still disliked that).

Regarding your final question about the Centro vs the 700p, I would say the main differences are size, battery life, and keyboard. The Centro is smaller, has a harder to use keyboard, and the battery life is okay at best (the battery is 40% smaller I believe). The Palm OS is pretty much the same besides a built in IM client on the Centro.