The taxi will cost you no more than 10-15 euro (per car, not per person). I presume you are looking at the Hertz location on Via Cisanello, not the Hertz at the IKEA, which is very close to the airport.
However, the price difference you were quoted is exorbitant. Generally renting from the airport shouldn't cost more than 40 or 50 euro extra (same vehicle) for a two week rental.
I suggest you do the following. Use a consolidator:
www.autoeurope.com
www.kemwel.com
These are two companies part of the same corporation but prices vary so check both.
They work with major rental companies in Europe (Europcar, Hertz, Avis Budget, etc)
Make sure to select the option No Deductible insurance rate (aka No Insurance Excess), for piece of mind. It's not that much extra.
Enter dates, location (Pisa All Locations, in your case)
I tested one week in August (13th to 20th) and Europcar comes substantially cheaper than Hertz.
Most of us contributors on this website use the above consolidators. I've used them for a few years with no complaint. Of course the actual rental company (Hertz or Europcar) occasionally tries to add the extra charge ( especially the fuel charge, even when you return it full), but that is another story. Keep all receipt for you and take a picture of the fuel gauge when you return the car. You'll need all documentation in case you have to dispute the extra charge with your credit card.
Watch out for speed limits. They are enforced by hidden cameras. If you get caught the fine will arrive later courtesy of the rental car company.