While all this is certainly useful and true, there are other ways in which you can do it on more of a budget. You might also want to factor in the cost of a dedicated PC for this too since that software will probably use up a fair chunk of CPU cycles.
My security system is far more budget but nevertheless gets the job done. I've got a cheap old 17" Intel iMac with a broken screen hidden away in the garage. I then have a 10m USB extension cable (with a signal booster) going across the garage to a Logitech QuickCam Vision Pro that's beet waterproofed and has been installed in my front porch. I then run Periscope (http/www.freeverse.com/apps/app/?id=7002) on the iMac and have set it set up to take a camera shot every time movement is detected and save it to disk. You can set it to email you if movement is detected if you like or to ftp the shots up to a server but an outside camera detects soooo much movement that you'd quickly fill your inbox.
As an added security measure, when I go away, I run Periscope on my desktop machine as well although this time its set up to email me if movement is detected (which is fine, since its indoors).
This set up cost me $25 for the Periscope software per machine and about $100 for the outside webcam (although they're much cheaper in the states). The usb extension cable was about $15. My desktop machine already had a webcam so I didn't need anything there and the garage machine cost me $170.
So all in all, my setup cost a little over $300. I plan on adding an extra webcam to the back of the house via another USB extension cable or via a USB over CAT5 adapter, connected to the garage machine, which would cost me roughly an added $140.
I can also stream/record the video from the webcams by splitting the signals with CamTwist and by using QuickTime Broadcaster / YouStream. The novelty of that wore off pretty fast though and I choose to save the CPU time for other more useful things.
Of course, I'm sure there are similar camera motion monitoring software packages out there for Windows and Linux so you could easily build a cheap dual core garage computer for what I paid for my one and then the other components would cost roughly the same.