My wife and I are going to Northern Italy next summer over a two-week period and are looking for an agriturismo to stay at over a 5-night period while we explore the region and its offerings in a car. I was looking for suggestions for agriturismo near Siena or Montepulciano - hopefully with a pool, breakfast, and potentially near the cities or close to walking distance if possible. I know this is quite the list, but will take all suggestions that people love for hopefully $250 or less per night - trying to get ahead on looking so we can book later this fall before it fills up.
We have stayed here; it is close to Siena and has not one pool but 6 of them. Cost is well under your stated price.
https://www.mulinodiquercegrossa.it/en/index.html
It is not exactly your quiet little rural agriturismo, however. They take day guests for the pools, so there can be lots of people on weekends.
This place has added a swimming pool since we stayed there and is about a 15 mintue walk to one of the gates to the old centre. It is quiet, has parking, small apartments and rooms. Lovely host.
Agriturismos by their nature are farms, so they are not usually (at least in my limited experience!) within walking distance of the towns. We just got back from Italy yesterday and spent six nights at a fabulous agriturismo about 25 minutes outside Siena with a pool (but no breakfast--we simply bought yogurt and pastries from the nearby store since we had a fridge and full kitchen). It was relatively small (seven total apartments) and ideally based to tour the heart of Tuscany, IMO! The cost was so reasonable. We did have to book a week (I'm not sure if all agriturismos are like this, but I think a lot are). Our travel plans were such that we only stayed six (instead of seven nights) but the cost was so low for us (a family of five) we didn't mind paying the week rate. Our family agreed this six night stay was the best part of our two week trip (Florence/rural Tuscany/Venice). The host, Silvia, was so literally one of the sweetest, most helpful human beings I've ever encountered. I don't think I've ever had better hospitality anywhere I've stayed.
We stayed a week at La Bruciata 3 km outside of Montepulciano. It was fabulous. Included is dinner on your first night of arrival. The breakfasts featured products fresh from their property. Laura, the owner, was the perfect host, even going to the hospital with a guest with a medical emergency to help translate. Homemade doughnuts appeared on our table one day! The apartments were recent construction, but felt like 18th century Tuscany. It was a perfect location for exploring that part of Tuscany and Umbria. The pool was a welcome respite after a day of exploring. I would love to stay there again.
https://www.agriturismolabruciata.it/en/