From April 30 through May 2, 2014, a friend and I stayed at an agriturismo in Caltanisetta province (right near the center of the island), Azienda Agricola Silvia Sillitti: http://www.sillitti.it/3_agrifarm.htm. This was a beautiful place, with rolling hills in every direction; it's what I picture "rural Tuscany" looking like, at a fraction of the price and with no crowds. There's a pool and both covered and uncovered terraces, so it's a great place to just relax, and its central location means that much of the island is only 1-2 hours away by car. A car is an absolute necessity to stay here, unless you just want to rest on site (she will pick you up at a nearby bus station, but then you wouldn't be able to go anywhere else). The only negative - no WiFi (but again, cell phone reception for both voice and data was fine).
Silvia herself really made the place memorable. She speaks good English and was a perfect hostess. We had a two bedroom, two bath unit with a kitchenette (burners and small fridge, but no oven) for €40 per night per person. Breakfast was included, and we paid €25 per person each night for her to make dinner (we didn't fancy driving rural roads in the dark). She took great pride in her dishes, made whenever possible with organic products including her own olives and beans. And her guests were very varied and it was fun to socialize with them each night.
Only one problem: in her directions, she had warned us that while a TomTom GPS would work, a Garmin would not. We thought this was an exaggeration - it isn't. The Garmin (even with recently updated maps) simply did not know the small roads right around her house. Getting there was quite a trial. She did suggest that we stop at a certain place at the main highway and call her to be guided to her house; we should have taken her up on the offer.
I'd definitely stay here again.