Since you use very little minutes and you don't need to use data and already have a GSM phone, go look at the 'Minutes' plan at H2O Wireless (h2owirelessnow.com). They are AT&T MVNO so they have exact same coverage with AT&T prepaid, and your unlocked GSM phone or even AT&T locked phone will work with their plan.
Their rates for the Minutes plan are: $0.14/min (if you refill with $10 or $20 PIN which expires in 90 days) or $0.06 or $0.05/min (if you refill with $29.99 or $39.99 PINs, respectively, which expires in 30 days). Texts are flat $0.05/txt with the first text of the day $0.10.
Details are here:
https://www.h2owirelessnow.com/pageControl.php?page=plans
If you already have a GSM phone, all you need to do is get the starter kit from their website for $10 and then you can activate it through the website or calling in. The kits are available also at BestBuy and other retail stores for the same price, but the gems are the resellers you found on eBay that are selling the starter kit for less than $6. Remember that activating a Minutes plan gets you a bonus of $7 of minutes so they're basically paying you to get a line activated.
I've been with AT&T prepaid for more than five years with even less minutes usage per month than you and it's crazy the amount of money I'm saving with them compared to GoPhone.