I've been tracking low cost approaches to computing over the past few years, especially because the cost of personal computing has increased so much.
Ahh, but today a computer without net access is almost a doorstop -- and net access is not cheap. It's not hard to find families paying more for monthly communications than they would for payments on a new car. Even though we are relatively cheap (rabbit ear TV/no cable, 2 netflix DVD plan, kids are H2O wireless voice-only mobile, war on AT&T) our family's yearly all media communications bill is probably close to $3,000 . That's unaffordable for many families. It dwarfs even the 4 year amortized price of a MacBook.
So how low can one go and still be able to do basic email (Gmail), basic messing (iMessage or ad-supported), maybe some Google Voice or VOIP , perhaps a bit of Facebook? Based on some headache inducing research from a recent trip to Canada i think one could do it for about $170 a year including Canadian taxes - not including the cost of acquiring a 3GS (0-$150 depending on friends and family)
Here's how I get those numbers:
- Buy a $100 voucher for a Rogers PayGo by-the-minute plan. This will provide emergency mobile service for 40 cent/minute and expires after 1 year. There's a $1 fee each month for "911 access".
- Pay $10 a month for 100MB of data. 
- Get a GSM iPhone 3GS Need iCloud services for basic backup and computer-free configuration so need iOS 6.
 Guesstimate, includes some media costs which aren't purely communication costs.  Google Voice is not VOIP of course and it's also not available in Canada.  I can't rule out other hidden fees that may hit. Mobile carriers are evil.