Anybody has any idea how much will it cost me (roughly)?
Thanks

If you fly by a regular airline, around Eur 200-300. If you fly by Easyjet, it can be anything starting from Eur 30 one way, depending on when you buy it and which flight. You can not fly to Bern, only to Zurich, Geneva and Basel. From Zurich or Basel to Bern, if I remember correctly, it is about Eur 30 return, from Geneva a bit more.

Helvetic.com flies Amsterdam - Zurich for € 99.- (roundtrip) + € 33.- charges. The train ticket from Zurich to Berne would cost you about € 50.- (roundtrip) and would take a bit more than an hour.

EasyJet flies Amsterdam - Geneva starting at € 15.- one way excluding taxes and fee. The train would take you from Geneva to Berne in 1h 45 minutes and would cost you about € 65 (roundtrip).

Thank you guys.
You've been very helpfull

What about train ticket?

You can buy the train ticket at the train station before boarding on the train. Prices: see above.

If you mean the train ticket from Amsterdam to Bern, I do not remember the price, but it is certainly more expensive than any of the budget air companies. Smth about Eur 100. Also, it takes about 10 hours and not worthwhile (unless you want to see many landscapes in the same day).