I have been looking at the bahn.de and think I've found the train we would like to take. However, I'm nervous about transferring trains in Interlaken as there is a transfer time of only 5 minutes. Is that enough to get off one train, find the other and board? Also, can anyone tell me the approximate cost for this trip? I'm having trouble getting fares. Thanks!
Five minutes is plenty of time at Interlaken Ost. It is a small station, and if I recall correctly, has only one platform with the 2 tracks. You won't have to walk far at all. Move to the doors and be ready to get off the train when it stops, and you'll be fine. You can get the fares for the Swiss portion on the Swiss rail site, rail.ch (in English). It should show you the track numbers at Interlaken Ost as well. That will show fares as far as Milan. You can use Trenitalia to find the fares from Milan to your town in Cinque Terre.
The standard 2nd class fare for Grindelwald to Milano is 95 CHF. However, if you book well in advance on the Swiss Rail site, you can get a discount fare of 45 CHF. For Milano to the CT town of your choice you may be able to get Mini fares for any legs that are on IC trains by booking each leg in advance on the Trenitalia site. There are no discount fares for regional trains.