For 0 euros check couchsurfing.org
Enough people to rent a flat? check airbnb.com it might get much cheaper and more cozy from hostels and hotels.
To and from Pisa airport?
The bus leaves Pisa airport and arrives at Florence directly as mentioned above. More info here: http://www.terravision.eu/
- Don't think about it too much, prices get higher :)
- Double check your hand luggage size according to the company's limitation!
- Remember to check in online and print your tickets!