What about trains from Paris--Avignon
Paris--Bordeaux, Bordeaux--Avignon
What does it cost?
Is it best to purchase tickets before you leave home or in Paris?
Where do you purchase tickets in Paris?

You purchase your tickets from the www.voyages-sncf.com site and print them yourself if you want to save money. By choosing the proper date far enough ahead of time, you can pay as little as 19€ for Paris-Avignon, as opposed to the full 2nd class fare of 92€.

There is a British flag in the bottom left corner of the site for the English language version.

ditto and if you have any problems with the site expert help from folks like Kerouac, TimS and MorganB (who has a whole thread walking you through it all) will surely help you do it. Tip is i guess to try to get the cheap online fares as soon as they come on, 90 days ahead i believe.

If wanting flexibility consider the France Railpass if taking all three of these fairly expensive on a walk-up basis trips. passes can be used on any train but you do need to make a reservation for 3 euros on TGV and CorailTEOZ trains but pass is good on any train for train fare. Raileurope and others market these in U.S. and not sold in France i believe as French can't use them. I always recommend calling BETS (800-441-2387; www.budgeteuropetravel.com) if any questions because of their expertise. www.ricksteves.com also has good info on train travel and passes.

I echo Kerouac's suggestion about purchasing your tickets online. The SNCF website can be a bit quirky, I found, but have patience. One key to this is not to say you'll collect your tickets in the USA when asked - Say France and then use the print your own tickets option. If you say "USA", the site may redirect you to RailEurope, which has an "exclusive" on sales in the US (an exclusive to sell higher-priced tickets apparently)

Also, if you know a little French, have a French-English dictionary or can use an online language translator, try using the French portion of the site. I swear I got more routing and ticketing options in French than in English.

<as little as 19€ for Paris-Avignon, as opposed to the full 2nd class fare of 92€>

full fare 92 euros or about $140 just for a one-way Paris-Avignon lets you see what a bargain the France Railpass can be for flexible travel as for $195 (saverpass 2nd class) you get 3 days of unlimited travel in a one-month period and can add on extra days at just $28 a day! So if taking several different train trips the per day cost of the pass drops dramatically.

flexibility is especially desired for folks landing at CDG and wishing to take the TGVs - non-refundable non-changebale PREM fares dictate you put a lot of fudge time between plane and train. If you make a reservation ahead of time with a pass, if you're late you at least have a chance of taking another train and just being out the reservation fee.

personally i demand flexibility when traveling but if you itinerary is set in concrete than the online fares make $EN$e.

youth pass for those under 26 is $167 for the 3 days and just $25/day after.

Passes are much better bargains in 1st class - $228 p.p. for 3 day/1 mo with extra days $34/day and first class always has its benefits in more leisurely travel.

Thanks to the efforts of Fodorites who are experts in helping with the sncf site it's easy to compare and see which is best for your needs.

And note that for some weird reason the France-Benelux pass, good over a 2-month period is actually cheaper in some versions than the France Pass alone!