Read through recent posts in the various Forums here and you will see possible estimates of cost-per-square-foot, but really, so much depends upon the area you are in and the nature of your site and the extent of your design. Local codes would have to allow what you want, and not all do, since older fire safety laws usually require a maximum number of seats per row and that such be anchored to the floor. If such codes apply, can you get them changed?
As you surmise, being a restrateur is not sufficient to know the exhibition business, so it might pay to first become an employee of a cinema and work your way up to manager as the best way to learn the business.
You mention a single screen, and as the many posts here make clear, they are rarely profitable in these days, and the fact that you have lots of competition only makes that idea more impractical. You would have to have the most wonderful food and service to compensate, and that is difficult to work out with those wanting an atmosphere quiet enough to enjoy a film. Create too much of a party hall, and the place becomes lots of noise with a film only incidentally in the background. It may be hard to get a contract with a distributor or booker under such circumstances, yet there are 'dinner theatres' in some communities that make it. You might ask Jay Hollis, owner of the sofa-seat ROSEBUD CINEMA in Milwaukee how he has been doing it for six years in a small, converted movie palace. Search for his web site on
www.google.com Best Wishes. Jim