Where are you coming from? The airport at Pisa is larger than the one in Florence and has flights from outside of Europe come in to it, while the Florence airport only has flights from other European countries (the takeoff/landing strip is not super long and cannot handle planes over a certain size).
Start searching on skyscanner to get an idea of costs, but generally I have found also checking expedia, kayak and other flight sites helps me get an idea of what are the going rates. Lately, I've actually found best rates booking on airline company. Make sure you search on browser without cookies on, so that your searches are not traced. I keep hearing that if your history maintains info that you're searching, every time you go back and search again the rates go up.