You get about 1 mile from the battery itself in EV mode which sounds quite small. However, leave it in normal and the vehicle will use the battery at low speeds and then the engine will kick in at higher speeds, recharge the battery whilst also braking will cause it to regenerate. So it essentially does this over and over again.
Lifetime of the battery is difficult to gauge but Ive had Lexus hybrids since 2012 and never had any issues with the 450h. Its a very reliable unit. I sold my 3rd gen RX450h with 55k miles, my current RX has 47k miles on it and the IS300h we had had 18k when sold so I honestly wouldnt worry about the battery. Not sure on the cost but I dont think many have been replaced whatsoever.
I think MPG wise im getting about 25mpg in town and can get 35-38mpg on a steady 70mph run