Having grown up about 10 minutes from Capraia, I would recommend that area in general as "slightly less" expensive than the Florence to Siena corridor. Montelupo is very close and probably the train station that slinky is referring to, and Empoli has lots of direct trains to Siena and is only a few more minutes away.
The closer you go to the sea, the cheaper it will be. Lucca is great, and the whole area is less busy, and there are some beautiful, if somewhat remote, areas on the mountains. If you hit the seaside towns, of course, the price goes up again.
It's a challenging time of the year to do the big towns like Florence and Siena. Aside from the fact that Florence is beastly hot, and the most humid city in Italy in August, so much is closed, including my favorite restaurant there. We've sworn off going back there in July/August. I do understand that it's difficult sometimes to find a good time.
Small towns around the central and southern countryside can be significantly more pleasant to hang around in the evenings. The area around Capraia/Carmignano, Vinci, San Gimignano, Montalcino, Pienza, etc etc etc are beautiful, and you'll sleep better in the evenings

There are smaller, less popular towns in similar areas like Montespertoli, Certaldo, lots others we could list, which would be a LOT cheaper. Typically, the price driver is going to be either how popular of a place it is in itself (Montalcino or San Gimignano would fall here) or how convenient the location is to a train station (Capraia is really good for this). If you stayed in Certaldo, you would be doing a lot of driving, but the old town is wonderful, and active and lively for the Mercantia festival, usually around July end (haven't been in 11 years myself).
I'm going to stop going on. If you decide to go let me know, even by PM, and I can give you some more detailed pointers in terms of where to stay, what prices etc etc.