Traveling from Interlaken to Venice

Probably the trains: Interlaken to Spiez, then under the alps via tunnel and thru to Milan, then Milan to Venice.
Approx. costs here:

https://www.ricksteves.com/travel-tips/transportation/trains/cost-maps

There are not really cheap and expensive routes. There is the quickest route, and different Italian trains have different prices depending on demand and how early you buy.

Go the the SBB (Swiss Railways) website: https://www.sbb.ch/en
and enter from: Interlaken Ost, to: Venezia S. Lucia, date: Tue, 22.05.2018 and a time.
It gave me lots of trains, the first of which was:

Interlaken Ost dep 08:00, IC 61, Spiez arr 08:21
Spiez dep 08:36, IC 8, Brig arr 09:11
Brig dep 09:44, EC 37, Venezia S. Lucia arr 14:40

Tickets for these trains are currently selling at CHF 125
Trenitalia ( http://www.trenitalia.com/tcom-en ) is currently selling tickets fro the train from Brig to Venice at €59. Doing a split purchase Interlaken - Brig on SBB, and Brig - Venice on Trenitalia will probably save you money.