Here's the averages for
Skype data usage. Figure around 3MB per minute, or about 3GB of a month of calls lasting ~30 minutes a day.
Any form of network based, peer-to-peer VoIP that doesn't use POTS bridging is definitely the cheapest way to go all around so long as the data is cheap and the latency is relatively low. Not crazy about Skype, but it works without having to know what the bleep you're doing. There's more efficient speech codecs with VoIP services such as leaner iLBC settings, G.729 and GSM, but voice quality suffers some for the reduction in bandwidth. That said, you can theoretically get some VoIP services down as low as 500kB/min with the right settings... but again, complexity of configuration. Despite the security and privacy concerns with Skype, it's still the easiest load-and-go app for this situation if you're able to do data on both ends. Of course, going GSM wireless, you're probably looking at relatively high latency issues which could cause call quality problems.
Any sort of Peruvian POTS termination isn't looking too promising, from a cost effective nature, especially not with mobile services. If a really cheap Lima landline were available, you could call through
VOIP.ms to her for 2.67¢ a minute, or about $24 a month (around the same price as her wireless data service that'd be 2/3rds of the way to what you'd need), but that doesn't factor her costs... and is out of reach given her going to Moquegua anyway. Rural and (most) mobile service, even at its cheapest (as I'm sure you're aware) runs close to 20¢ a minute otherwise with VOIP.ms, which isn't to say there aren't other provider options.
Truthfully, unless a wired network connection could be had, I wouldn't chase down the rabbit hole of Skype-to-Skype too heavily unless you already know what to expect with call quality given the situation. Depending on the monthly cost of the cell phone, and the international rates to that prefix exchange, that might still be your most affordable option, and either calling card or VoIP to POTS termination is still likely to be your best bet. I'm not too crazy about
VoIPVoIP, but they have some of the cheapest per minute rates to Peru in general listed, with some rural and mobile termination as low as 1.9¢ a minute, and if you're willing to do a softphone like Skype anyway, you could just use it with
csipsimple on your end with your smartphone and your home internet connection, or
X-Lite on your desktop. Cutting out the middle man and the POTS to POTS termination this way using VoIPVoIP or VOIP.ms will probably prove cheaper than Google Voice or regular calling cards as middle-men are being cut out of the equation. It'd take a little work, but the support for the softphones are there by both providers.
Best advice I can offer. Sorry it's not more complete.
