i was there twice since the opening about 4 weeks ago.
first time:
i had the swiss burger (cheapest of the 3), with cheese (cheddar, you can choose from 4) and bacon (makes everything better

) and fries (cost extra as well as the bacon/cheese or egg if you want).
i wanted it medium, but when i got it it was welldone (a bit dry). in the end the bill was around 35 CHF, quite expensive. so my first experience with helvti diner wasn't the best.
second time:
i had the wagyu beef (it's liek kobe) burger (most expensive). with all the extras and fries it was about 50 CHF. expensive yes but the BEST burger in town in my opinion! i really like the buns and the beef... you have to try.
oh and almost forgot, great service!
my advice: always call and make reservation, its full (lunch/dinner time). or you go at 3am when it's still open

dont be cheap! try the wagyu!