We wouldn't have gone to the restaurant if we had known the prices.
This is correct. it's the 3rd conditional rule, where the if clause uses the past perfect, and the main clause uses the perfect conditional. You are talking about a time in the past.
We wouldn't have gone to the restaurant if we knew the prices.
Although we say this, it's grammatically wrong. To correct that I would say "We wouldn't go to the restaurant if we knew the prices". This is the 2nd conditional rule which uses the simple past, and the main clause uses the present conditional. It's a hypothetical situation in the present.
Hope this helps!