An inexpensive approach would be some network camera, e.g. Panasonic (or Axis if you spend a little more). So let's say $60-200.
Hook it up to Vitamin D, analytic software that distinguishes people vs. objects. Can send email or pic to your cell phone. Demo version for one camera is free. Pretty neat software, not perfect but my entire system is based around it.
It will generate some false triggers if it rains or is very windy. So easy to review footage.
Tried the D-link and it was dreadful, returned to best buy.
Another approach would be PIR based motion detect built into some cameras. Panasonic and Axis both have models with that approach, among others I'm sure.http://www.axis.com/products/cam_m1031w/movie/index.htm
It's not going to look like the pictures they show but you get the idea.