I often park on public streets in suburbs/villages next to the airport (preferably near a bus/metro/train stop that takes me directly to the airport!). I make sure to park legally. But, even if my car isn't particularly expensive, I wouldn't be very comfortable leaving it unattended for an entire month. I'd probably still go ahead and do it if the savings were big enough, but not before double-checking the prosperity of the area and lighting, traffic etc of the particular parking spot.