I realise this question is very old, but there are enough views that answering seems like it might help people.
You don't need to do this with the Jungfraubahn, that's probably the most expensive way. Take the normal train (twice per hour, direction Interlaken) to Zweilütschinen and change there to the train to Grindelwald. Should take about 35 minutes in total. almost 3 years ago