We are driving to Siena Wednesday. I hope to arrive around 10am. Where’s the best place to park? Cost isn’t an issue.
Just about all of the Centroid Storico is a ZTL. There are numerous parking garages in which to park outside the ZTL. Just open Google Maps and type in “Parking Sienna” or use Google search.
I had trouble on the approach, because the GPS tried to route me through the center of the city to get to the perimeter parking I had entered as our destination. There are large (uncovered, I think) lots. Here's one additional resource:
https://community.ricksteves.com/travel-forum/italy/siena-parking
I always park at the stadium parking lot:
https://goo.gl/maps/jvzqxP8KcQQY4pSQ7
Save it in your Google Maps. Coming from the freeway exit at the SIENA OVEST exit, no matter which direction you come from. That is the easiest route.
The entrance is next to the San Domenico Basilica. See the image below. The entrance is on the left where you see the P sign.
https://goo.gl/maps/SmDdStsRg2Tr3vJFA
Always keep an eye for signs to the stadium, which is a soccer ball symbol
https://goo.gl/maps/mmXGS5dKfpmNM2r88
The target symbol means "city center". The Soccer symbol means stadium. The P in a blue sign means 'Parking'. Those symbols are valid throughout Italy.
There's an underground lot at the train station that usually has plenty of free spaces. Cost is 2 Euros per day, and it's located on the main highway getting in and out of town so getting on your way is pretty easy. It's well clear of the ZTL's too.
A taxi from there up to the Campo cost 12 Euro a few years ago during our last visit - which we thought wasn't bad for door to door service with luggage.
From the train station there are a series of escalators that take you up the hill, then you walk a few blocks to the Porta Camollia, then a nice interesting walk of about 15 minutes to the center of town. Take a taxi back.
Another place to park with escalators to the top is Parcheggio Santa Caterina. It's a lot, not a garage. The escaltors (risalita meccanica on the Google map) are inside a building just a little farther up via Esterna di Fontebranda. The escalators will take you to a spot that's closer to the center than the train station plan.
I'm just giving you two alternatives to Roberto's recommendation --- I would take his advice over my own any day.