I get by quite nicely sharing a 500-minute plan (one of the lowest available) between three family members for $60 or so a month. By taking advantage of in-network calling, we routinely come in with minutes to spare and I personally no longer have any need of a landline. Most providers are now also offering free minutes with up to a half-dozen out-of-network numbers - we haven't used that feature yet, but I imagine it'd only put us further below 500/month.

The best part is that we've had the same plan for about four years, and are no longer under any contract. When the time comes for a new phone, I've had a better experience shopping for a specific model on eBay than wasting time with buy-one-get-one offers from the provider.