Travelling to Bergen by train is really only practical if you're coming from Sweden, as you would have to travel via Oslo.
Coming from continental Europe is slightly more complex. There really are few alternatives to flying, but on the other hand, there are direct flights from several European cities, in particular in Germany. A very common connection is flying via Copenhagen, Denmark. But a ferry connection between Ijmuiden in Holland and Bergen has been announced as starting this summer.
The price for eating out isn't much higher than what you'll find in most other parts of North-Western Europe. The higher cost of living in Norway comes mostly from the prices on basic food articles. This is much more noticeable to someone running a daily household than to someone eating out. Which is what most people visiting for only a few days will be doing anyway.
There is a large number of restaurants in the immediate vicinity of the convention site, including both vegetarian, Italian, traditional Norwegian, and Chinese.