why would people buy a specialist device just to spend money when most people object to having to pay $10 to obtain a pre-paid debit card upfront. most men object to spending more then $30 for a leather wallet. so paying more for an electric gadget that only has the function of spending money at limited range, relying on other local people to be nearby to relay the 'messages' around if flawed.
in this day and age its all about phone apps. utilising Near-Field, bluetooth, wifi,sms, 3g/4g internet makes there is no need for a peer to peer specialist device with limited distance of use. when our own smart phones can do the job nicely. all it requires is a good stable app to link everyone together without it using up peoples phone balance and still remaining secure so that others cant gain access to the other phone features outside of the app, such as addressbook, phone dialer, etc.
I suppose people might buy it exactly for the reason you are mentioning... money. What's the cheapest phone plan out there to be able to text someone? If an ad hoc network combined with gateways and servers could pull it off then a small initial investment would easily be justified if you used text. The problem as I see it is needing other users for the ad hoc network to work.
The ad hoc network should be paid to pass on packets. Each node can add a bitcoin address and the receiver would pay each node. Those who didn't pay up could be punished prioritywise. Perhaps instead the route can be established and then the sender can reuse the connection and pay in advance. Either way we won't have time for confirmations, so we will just have to punish bad actors. We just need some kind of routing algorithm with fee negotiation. A nontrivial task for sure, but doable with enough effort.
We would then see people with cellphones set up routers. The data for bitcoin transactions and text messages is tiny and would barely affect their data usage, and they could set their rates however they like. Data consumers could also set limit on what they would like to pay.