seat reservations

Are you talking about 1st class vs 2nd class? If so, 1st class is about 2 to 3 times more expensive than 2nd class. The cost of either class depends on how far you are going.
Also, regional trains are far less expensive than the Eurostar trains, but are also a lot slower because they stop more often.
It seems you can't buy train tickets ahead of time on trenitalia's website; we just bought them at the train stations, either at the windows or using the kiosk/self service machine.
When you get your ticket and before you board the train, BE SURE TO VALIDATE IT in the "time clock" looking machines before getting on the train! Failure to do so will result in a 50 euro fine that must be paid on the spot to the conductor! I speak from experience: we were in a big hurry to make a connection and forgot to do it. It was a bummer.

If the train you are riding requires a reservation, it will come with your ticket. You do not need reservations for the regional trains. Don't forget to validate your ticket prior to boarding the train. I believe I read on this site that you do not have to validate a ticket that requires a reservation but when we were in Italy this past July we validated all our tickets just to be safe.

The cost of the seat reservation for long-distance trains (which have automatic seat reservations) is included in the price of the ticket.

No seat reservations are allowed or contemplated for commuter trains ("regionale" or "R" trains).

The hierarchy of fares on Italian trains is the following (in descending order of cost):

1. AV
2. ES-Fast
3. ES
4. ES-City
5. IC
6. ICN
7. EXP
8. R-Regionale

First class on "R" trains costs approx. 50% more than second class.

First class on other long-distance trains costs between 30% and 40% more than second class.

You MUST always "punch" your paper ticket for "R" trains in the little yellow machines by the rail track. While there is no longer a requirement to "punch" your paper ticket for any other trains for which there is an automatic seat reservation, you can nevertheless do so without consequences.

The only time you would pay a separate reservation fee is if you bought and used one of the rail passes that are available. If you look through Italy travel forums in most places, you will see that in Italy, a rail pass rarely makes sense. I think its either 20 or 25 euros per seat for the reservation in that circumstance. Raileurope.com may have the specific reservation fees listed, they are big sellers of rail passes.