Personally, I'd pay up to $100/month for a plan which includes (most importantly!) 1GB of data, 100-150 minutes, and a txt plan (TXT600 or TXT2000).
Any less than this (Steve Simms suggested a $40 plan) and the data provisions will be restrictive, and that's no fun! I don't wanna have to be constantly checking my usage for three devices! iPhone is all about the internet on the go, and not having to worry about overage charges. Have we not learnt from the days of horrific overage on Jetstream?
Let's hope so.