Most prices remain the same. But at SNCF, among others, prices can become exhausted, or go from 10 euros to 20 euros.

If its less as 10€, it usually means the set fee for (optional )RES and this is the same for anyone any time. Or around 20 for a couchette=basic ´sleeper´ IF they have it. But its 15 for a SEAT on the OeBB NIghtJet trains.

Higher prices MAY mean dynamic pricing =can rise if time goes by. But more likely is then the railway will simply not sell any passholders seats only, these quota are more of a thing to worry about if they apply in high peak season