Houses farther from Florence are not only cheaper, but also located in reach of less-crowded and more authentic destinations, such as Montalcino (home of Brunello wine), Pienza (fabulous pecorino cheese), Grosseto (Roman ruins, opera festival) or Volterra (Etruscan art).
Every area has its own character. The countryside around Florence and Lucca is manicured and sedate. South of Siena offers treeless hills, dotted with cypress alleys. Tuscany also has a beautiful coastline. Find a house you like in a catalog or online and draw a 50-mile circle around it (the most you'll want to drive in one day on rural roads). Look up the encompassed towns in a guidebook or online to see if they intrigue you.
There are more farmhouses than villas. These case coloniche are usually stone, with spacious ground floor rooms. Villas are more expensive, more palatial, and not as old (dating back to the 1700s). Note: Avoid renting a modern house; they are often hot and always charmless. And don't be afraid to ask how the house is furnished: you want antiques, not Ikea's finest.
Don't try to see all of Tuscany using your villa as home base. This is a chance to "go deep" in one small area. Visit the local churches, try every kind of meat at a nearby salumeria. If you rent a house near Montalcino, alternate day trips to nearby Scansano and Pienza. To keep prices down (and camaraderie up), include friends and it can cost about $150 per day per person.