Screen to car ratio: Ideally, you want to have one big lot and two "move over" lots. I would suggest 500 cars for the main screen, and two smaller lots at 300 cars each. When the weather is good, and there are strong enough movies in release you will fill all the lots. But the weather isn't always going to be on your side, and having three films kicking ass in release all at once happens only on occasion, so having all screens at 500 cars each would be a lot of wasted real estate.
In the U.S.A., the cost per sq/ft for parking area will depend on what materials you use, asphalt or stone. You should figure at least $200K for a 300 car parking lot for stone, triple that if you want to cover it with asphalt. Your local contractor can give you a more accurate cost per sq/ft.
Inflatable screens are not adequate for drive-in use, the bottom of the picture area is too low from the ground, so the sight-lines are poor, which will cause problems and complaints from customers. You need 17' to 20' of clearance from grade to the bottom of the picture area, otherwise the people will not be able to see over the vehicles parking in front of them. Inflatable screens work OK for "lawn chair" type movie presentations, not for permanently constructed drive-in theatres. Also, you really don't want to know what the screen is going to look like after you inflate, deflate, store, and over and over again for several weeks. Get a strong breeze during the show, and your movie screen will be two towns over.
Good luck!
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