Hi Blair,
Great question. Yes counting numbers at what you consider to be an equivalent establishment could help. Some other thoughts;
– have you used dr google? Often great insights online.
– contact the restaurant association and see what guidance they can offer you.
– get a mentor – find someone who has done this before and is happy to share some experience and wisdom. Most people are really generous so work out who your dream would be and give them a call! You can find a lot of names on public record or through articles in publications like The Denizen

The cost vs sales question really needs some sector expertise so hoping some one else will come back to you on that one. Thanks! Vicky