You can set up a system for close to nothing. All you need is a computer, webcam, and free software.
iSpy is free, open-source motion detection software. It can turn any video source into a motion detector. It even supports multiple video sources at once.
I setup an old computer with a webcam to do just this for me. I can even monitor my camera with my smart phone.
Oh, and of course the best part is that iSpy is free.
Check it out at iSpy.