Yes, it is expensive for Greece, even in Santorini. Oia is obviously the posh town of the island. But the view IS great (although often blocked by people taking pictures) and the food IS good, greek with an innovating touch. Try the stuffed olives... We had a lemony lamb filet that melted in the mouth. The only weird thing is there was no fish on the menu.
My advice is to book a table for an hour before the sunset on a "cloudy" day. And make plenty of time to get there: it is located at the very end of the main pedestrian street and hundreds of people gather in the vicinity to watch the sun go down, blocking the way to the restaurant. It took us half an hour to get there and we only just made it in time.