Fairly common question... probably best answered as a relationship between total cost and your personal experience. If you have no background in purchasing, installing and maintaining projection and sound equipment, plan on paying 100%. The newer you go, the more reliable it (should) be. If you need to go new, figure it into your tax planning.
If you have some level of knowledge, you can trade it off for good used equipment. If you can do any of the installation and maintenance, you save that way, too. If you're experienced in "all things theatre", you can probably build 4 or 5 booths for the average cost of one. If you're REALLY good, you can take this on and learn as you go. Barry Floyd built his second drive-in screen from scratch. It turned out pretty good, from all accounts.
Assuming you're in the first category, get ahold of two or three trade mags and check for nearby equipment suppliers. You can also google for much of this. As for bookers, we have one or two on this forum. You might also call up a studio or two and see who they deal with out of your area.