http://www.ferroviedellostato.it/ (and click english)
you can check everything u want-
Rome central station is: ROMA TE (roma termine)
Florence main station is: FI.SMN (Firenze Santa Maria Novella)
pls check carefully the station of departure and arrival.
Eurostar and Alta Velocità is the fastest, but it should be around 30 euro each, more than 100euro for 2 is impossible. if u want cheaper, take Intercity (IC), or the cheapest "Regionale" (R)

after entered the place of departure and destination, u will see a list, you should look at the 5th col, there will be "ES", "IC", "R"....click the one want (the last col), enter the page and choose "standard" then u will see the price.