If I am staying in lauterbrunnen, and want to go at afternoon train at 3:30? Approximate cost for the day? Any help? This will be happening on Aug.21,2014.
And what if I am a swiss pass holder?
Have a look at THIS website as it should provide the information you're looking for (Base price without pass is CHF 197.60 return, 2nd class). THIS website has information on the Swiss Railpasses, and what they cover. I believe your Pass will provide a 25% discount, but hopefully one of the others can confirm that.
Be sure to check the weather before you make that trip, as it may be clouded over at the summit (or course you could still see the indoor exhibits and the Sphinx Observatory). I'd probably make that trip a bit earlier in the day.
The best time to visit Jungfrau is first thing in the morning, later in the day it tends to get clouded-in on most days.