This subject was discussed several times in the past. A review of the archives by setting the search date to all and including all forums/threads should provide several minutes of reading on the subject.
FWIW
We've been using the Costco cards since they introduced them and a copy of their calling / refill # is included with the calling card # in each of our family PSK's as well as on a small label maker tape wrapped in a spiral around the batteries in our ARC AAA and AA flashlights that are EDC. We've used the callling card several times when cell service and other providers were not available. The Costco card does need to be used every so often (yearly?). At the time we purchased it, it was the lowest priced per minute card available and it can be refilled.
SIDENOTE: I've included a Costco "CASH" card in each of our USK's ($30-$50) which has come in handy from time to time. These can also be used at the Costco stores (having gas stations) to purchase gas in addition to the normal purchasing in the store itself. Like the phone cards, these are also refillable. In some areas there may not be a Costco nearby for you, but there are many other brands of cash cards now...everybody is on the bandwagon it seems. Hope this helps.

Regards,
Comanche7