First of all make sure you go on an absolutely clear evening/night. If you cannot see the top of the mountain from Vancouver because of cloud cover, you will not see the lights of Vancouver from the top. If you are going up for the shows and attractions, make sure you realize the time span between the last of the show and your dinner time.
I have eaten at the Observatory. It is not a particularly large space, not bad. There are half a dozen tables positioned along the windows, after that you would not get a window view. I sent my meal back because the food was cold. Large dollars for food gone cold, hmmm. I can think of many many other restaurants that would surpass what The Observatory offers.
Right now it looks like the restaurant has a prix fixe offering which includes a main and dessert with wine pairings. Otherwise, the appies, mains and desserts are all priced individually. You will see the prices, so preciously spelled out in words, under each of the section headings.
Do you have a budget in mind? If you have a car, I would suggest you have a look at Fraîche and/or Salmon House on the Hill, both up high in West Vancouver. Then there is The Beach House at the Pier, right on the water at Dundarave. If they appeal, post for driving directions!!! The only down side about SHOH, is they put their window tables for 2 down at the far end, the rest of the window seats are for 4. But they do have their restaurant laid out in tiers, so you could sit at the next tier up and see out quite well.
Otherwise, for a nice dinner with views, you might even consider The Five Sails at the Pan Pacific Hotel.