This site is helpful. I looked into a few plans recently for use as an emergency phone, here's what I came up with.
Tracfone: You can get a 1 year service package, so you don't have to remember to keep buying cards every 2 or 3 months. The $95 package on the website includes a refurb phone and 150 minutes good for the entire year, but you probably should get a cheaper plan if you're a light user. Sam's Club is offering a $70 package that includes a new phone, car charger and 1 year service, but no minutes. (Some other retailers have the same package minus the car charger for $55-60). With any Tracfone, you can get a total of 20 minutes by activating online and then letting them spam an email address you provide. You can also pick up 100 minutes by letting an existing user issue you a referral *before* you activate your phone, you really need to do this to make the deal hotter. It's simple to do, the existing user just needs your email address. Other service plans without the 1 year of service are available, in that case new minutes need to be added every 60 days to keep the phone functional. I think you can use referral minutes every 60 days to fulfill that requirement.
According to online complaints, customer service supposedly sucks. When you roam you're charged 2X. They resell service from other providers. I don't know of an easy way to know exactly which provider you will get in your area if you buy Tracfone. There are threads here in Hot Deals, at f@tw@llet, and a yahoo tracfone user group, enough people live in your area that you probably could get an answer (as well as a bunch of referral-whores PM'ing and begging you for referral minutes, heheh) by asking around online.
AT&T: potentially the lowest monthly cost AFAIK. From the internet complaints, it seems that the overall reputation for AT&T's cellular network is not good, but that probably varies a lot by area. They're also supposedly being bought by Cingular later this year, whatever that will mean. You can buy a used AT&T phone on ebay or garage sale ($10-20?) or possibly at a local AT&T dealer, then set it up with their prepaid service. Check with AT&T to be sure that the ESN of the used phone is clean before you buy it (i.e. not tied to a delinquent account or stolen). They just recently changed their terms so that a minimum $10 (40 minute) card only has to be bought once every 3 months. Ignoring the initial cost of your phone, the monthly cost can be as low as $3.33. I never heard of 20/12 before, if I understand it correctly, it would seem to make AT&T even less expensive.
Virgin: runs on the Sprint PCS network, and I'm pretty sure you cannot roam off that network. Looking at their coverage map, it's fine if you plan to stick to cities and major interstates, but for my usage (emergency car phone) I'd personally prefer more extensive coverage. Initial phone cost is higher than some others. Monthly cost is potentially as low as $6.67 IIRC.
Verizon: I didn't look into this one as it's not available in my area. Verizon's network has a good national reputation. But at first glance it looks like the minimum monthly cost is substantially higher than some other companies.
Besides costs, I'd consider coverage maps, features, ease of use, service quality, etc., to determine what's best for your usage.
The site I linked to before is helpful for some of that. He seems to try to keep his site current. His monthly cost on Tracfone is a little high as he's basing it on the $95 deal, if you buy the less expensive annual deal it's a couple bucks lower. Also if you're good at acquiring and stringing along referral minutes with the non-annual plan, I believe you can operate a Tracfone more cheaply.
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