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Between Paris and Tours you can either take a TGV (highspeed train) or an "Intercitรฉs" (classical train). Both are cheaper when booked in advance. The full fare for a 2nd class TGV ticket is 65 EUR. Booked in advance it can be as cheap as 25 EUR. For Intercitรฉs the range goes from 15 EUR to 36.20 EUR.
You should also know that TGVs can sell out. If you are really unlucky and show up at a very busy day, there may be no more seats available.