Try this:-

http://www.rentalcars.com/SearchResults.do?dropCity=Siena&doMinute=30&location=143794&driversAge=25&doHour=11&searchType=onetomanysearch&locationName=Florence+International+Airport&doMonthYear=8-2014&doFiltering=false&city=Florence&puMonthYear=8-2014&chinese-license=on&puHour=11&dropCountry=Italy&puDay=9&filterTo=20&dropLocation=-1&doDay=30&dropLocationName=Siena+%28All+areas%29+&country=Italy&filterFrom=0&puMonth=8&puMinute=30&doMonth=8&doYear=2014&puYear=2014&filterName=CarCategorisationSupplierFilter

Thanks David, i was really looking to see if anyone had first hand experiences of specific companies and if there was indeed more charges. I booked a car in the states with that same website, but when i went to pick it up there were lots of hidden charges that hadn't been mentioned, as well as a huge extra for dropping the car off in a different city so wanted to avoid repeating that if at all possible! 

I'm living near Florence. You will pay more for leaving it anywhere other than where you hired it from... If you need to get to Florence from Siena after your trip there are buses every hour (90 min trip). Europcar in Florence is the easiest place for car hire (and central) in Florence. Otherwise anywhere in Siena should be okay - just check prices. Avoid hiring from Florence airport - more expensive, more hassle! BTW, if you hire in Florence centre you'll be able to drive in the city centre - otherwise it's banned without a local's permit.. but parking is a nightmare anyways, so best use the bus in the centre : (

Jonathan 

perfect, that's exactly what i was looking for. Grazie mille sir! :) I think we'll end up renting in Sienna as that is our first stop (we'll just take train from florence to sienna- those are super quick and easy) so will have a look what i find online. Cheers!