Built mine new back in 2003. Started off with one screen on 15 acres. Spent about $500k the first time around. Added a second screen in 2005 for a cost of $35k (equipment, grading & screen), then we expanded our concessions too earlier this spring for around $12k.
Rodeojack is right - and so is Ken. We tried several places before we could get the zoning passed to put in a drive-in. Lots of folks tell you they can't wait for you to build a drive-in, but they're not the ones who show up at the zoning meetings. The ones that show up at the zoning meetings are the ones who scream you're going to show porn to their innocent little children.
Like Ken said.... Zoning first, then land, utilities (water/sewer/3 phase electrical power / natural gas), buildings, ramps, screens, projection equipment,etc.
Don't even think about opening a drive-in if you can't have two or more screens. A newly constructed single screen drive-in is a really nice HOBBY, but it doesn't generate enough income to pay it's own bills. Although we turned a profit after 4 years, it took 7 years before the place would cash flow.
If you're just now looking at this, then sadly 35mm film will probably be gone before you get it open. Plan on $70k per screen for projection expenses. Screens.... several years ago they were $1,000 a foot (Selby), I don't know what they are now. My first screen was a Selby, my second screen is a pole-built structure erected by a pole barn builder from a farming community 20 miles up the road. Both screens work just fine.
You can count on it taking twice as long as you thought it would to build, it will cost twice as much, and won't make the money you thought it would right out of the gate. I'm almost 10 years in, and we're finally making good money... however I KEPT MY DAY JOB.
Good Luck....