As a general saying, the fares are fixed, you can find them on sbb.ch/en. There might be additional promotions on the Jungfrau website for early morning departures.
However, I strongly suggest not to buy any mountain excursion tickets in advance as you will not know what the weather will do! It is possible that you end up in clouds while you spent a fortune to get there. You should only decide the day before if it is worth visiting Jungfraujoch due to latest weather forecast.
The weather plays a major part when travelling in Switzerland.
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