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Train fares vary depending on the route, demand, when you book, your eligibility for discounts, and other factors. You can use the SNCF website to look up the options for your dates and times. With a booking sufficiently far in advance, there are some TGV options for β¬20, but bookings with less notice or more desirable trains will cost more.
The bus might be cheaper as well. There are bus fares listed on the SNCF website too (just switch the tab from train to bus), but you can also search on Rome2Rio, which will show you FlixBus buses too.