What should I expect to pay for a Eurail sleeper car?

In answer to your second and third questions...yes. Your Eurail retailer should have told you. Reservations can be made separately through Raileurope. Paris-Rome is anywhere from $37 each for a second-class T-6 couchette (six very small beds in a compartment) to $109 each for a two-bed sleeper (MUCH more comfortable). But that's only if you book way ahead. If you don't those costs at RailEurope could easily double. For that journey, you could also try SNCF, the French railway network, at 011-33-8-92-35-3539 to make sleeping reservations...they'll be cheaper. Unfortunately, you can't book just a reservation on the Italian rail site (Trenitalia), and they don't have an international number for bookings. But you might be able to book it through the German rail network (Die Bahn) at 011-49-1805-141-514. RailEurope for this leg wil charge $56 pp (for a T-6) and up. And don't forget, you'll need to have at least a four-country pass, since you'll be transiting Switzerland or Austria.

If Paris-Rome and Rome-Frankfurt are your only train trips, don't buy a Eurail pass. Booked far enough in advance (up to three months) at www.sncf.com, you can get discount fares: €35 for a six-bunk couchette, €60 for a four-bunk couchette, and €140 for a double sleeper. Click on the British flag.

For Rome-Frankfurt, make two separate bookings. Book the Rome-Munich night train at http://buchung.nachtzugreise.de (omit "www"), leaving Rome at 21:55 and arriving in Munich at 08:30. Booked far enough in advance, yo can get SparNight fares of €49 (reg. €140) for a four-bunk couchette and €69 (reg. €180) for a double sleeper. Click on the British flag.

Then go to www.bahn.de to book a Munich-Frankfurt ICE train, leaving Munich at 08:56 and arriving in Frankfurt at 12:05. The regular fare is €81, but booked well in advance you can get a Dauer-Spezial fare for €29. Click on "Internat. Guests."