Train from Rome to Venice terminal not Maestre

Time is about 4 hours from Roma Termini to Venezia Santa Lucia. Cost depends on how far in advance you buy your tickets; super-economy fares begin at about 19euro but permit no changes to the ticket (your seat is reserved when you buy the ticket). Economy fares permit some changes, with some costs associated. Base (full) fares permit changes, cancellations, etc. within a specific time period (sometimes one hour after the train has departed).

You can go to trenitalia.com, click on the British flag icon for English, and put a date in for 120 days from today to see the times and fares (Schedule for October may not be fully loaded yet, I haven't checked in a couple of weeks - schedules change in June and December).

The trains from Rome to Venice take around 3 and half hours and the price will vary depending on when you buy. There are two different train companies that run this route.

http://www.trenitalia.com/tcom-en and http://www.italotreno.it/?sc_lang=en

You will want to enter Roma (Tutte) as your departure station and Venezia S. Lucia as your arrival station. You can book on Trenitalia around 120 days in advance for the cheapest fares (super economy). The only downside to the super economy fare is that you are locked in to a specific train date/time. I see super economy available for around 20 euros on Trenitalia in July and Smart Fares on Italo for less than 48 euros.

You'll only enter Roma (Tutte) if you want to see ALL the available stations. Majority of travelers will begin their journey from Roma Termini the huge main train station. If that's where you want to travel from then enter Roma Termini as your beginning point.

Travel time on high speed Freccia train: 3h45min
Fare examples:
Cost of Standard Class (the cheapest) at Base (full) fare: €84 one way
Cost of Business Class at Base (full) fare: €114 one way
The previous replies gave you the info on Economy and SuperEconomy fares pre-purchased online.
If you intend to retain flexibility of changes and intend to buy tickets at the Base (full) fare, you don't really need to purchase online ahead of time. Those fares are always available on the spot, so you can buy them at the station when you depart.
Trains seats never sell out, however seats on the Standard class sometime sell out on the early morning trains at peak time. If you intend to depart to Venice before 10am and plan to travel on standard class, it is a good idea to walk to the station or to any travel agency when you are in Rome a couple of days before your trip and purchase your ticket then.
If you intend to depart later in the morning, then you can buy tickets on the spot the day you depart. The base fare will almost certainly be available on any class of seats on the late morning trains.
If you want to splurge on the Business class or higher, those seats are always available and never sell out, even on the early morning trains.
All details are on www.trenitalia.com.
There is also another train company you can use: www.italotreno.it