The biggest problem with most prepaid plans, IMO, is that the airtime expires if you don't add more money within 30-90 days. One exception I know of is Tracfone, who sells an annual card with around 250 minutes for ~$95. This isn't really much cheaper than other carriers' 30-90 day cards, but you have a much longer time window between chargeups.

Once in awhile I've seen Tracfone phones for around $60 with a year of service (but no minutes), but maybe only at places like Sam's Club.