I have an iPad and use it heavily, but only subscribe to the 250MB plan. In the US, free WiFi is widely available. LTE is a "plan B" for the rare cases where WiFi isn't available.
Like most tablet users, I mainly use it at work (over WiFi) and at home (over WiFi). When I'm out of home or office, I check email and occasionally browse or use maps. I suspect that 250MB/month will be perfectly fine for 80% or more of the buyers.