So this is where the cost comes in. The light switch style occupancy sensors are good for about ~100 sq. ft., or about a 15'- 20' wide strip about 5' - 7' deep. They are great at turning on the lights when you enter a bathroom, stairwell, or closet as long as you pass within range of a lightswitch.
Typically, there are limited paths in/out of a room, and you want the lights to come on when someone enters. I doubt you need coverage over all 900 sq. ft, just over the likely traffic points. This is the key here - the relationship of the light switch to room entry. If these are close together, you have a good shot at the simple solution. If you have a wall of windows to cover, you'll likely need something else.
If entry is possible at numerous points in that 30'x30' space, the solution may very well need to be an external occupancy sensor and high voltage interface like ADI quoted (their recommendation claims 1500' coverage). If you haven't already, I recommend thinking about how much sensor coverage you really need.
Feel free to chime in if you know, we'll keep going.