Florence to Siena on a Sunday in October

If you go to page 3 of the timetable below, you can see the Sunday and Holiday schedule (FESTIVO).

http://www.tiemmespa.it/var/ezdemo_site/storage/original/application/bd94ea39e3a4d7a839280815aa486191.pdf

As you can see there are buses departing the bus station (across the street from the Florence train station) at 14:10, 15:10, 17:20, 18:10, 19:10. The bus ride is about 75 minutes.
You can take a taxi from FLR airport to the bus station (15-20 min away) on Via Santa Caterina da Siena for 20 euro (plus a few euro for the luggage). The bus ride from Florence to Siena costs under 8 euro.
In addition to the bus, you can also take the train to Siena. The train takes about 15 min longer, and the Siena station is at the bottom of the hill (Siena's old center is on top of a hill), therefore the bus is preferable (bus leaves you on top), but the train is also an option which runs more frequently and until later at night.
Trains timetable below:
www.trenitalia.com
(from the airport you can select both FIRENZE SANTA MARIA NOVELLA or FIRENZE RIFREDI stations)

If you go by taxi all the way from the FLR airport to Siena, it's certainly convenient and ultimately faster (just over one hour drive) but the cost is in the 120-150 euro range.

In summary:
Taxi all the way: 140 euro (1+ hr)
Taxi+Bus: 30 euro (20 min taxi + 75 min bus + bus waiting time)

Mille Grazie Roberto

The Volainbus from the airport stops at the train station and then continues on to the bus station. It departs twice an hour on the hour and half hour, even on Sundays, and has ample racks for luggage. Cost when I used it in March was €6 per person; purchase ticket from driver. If you decide to use the express bus rather than a taxi, you can take it directly to the station from which you plan to depart (train or bus). Sunday schedule and stops can be viewed on the ATAF website.

Thank you!!!