How can I make a cheap, basic video surveillance system?

I just moved into a two-story house with a lot of poorly-placed windows. I plan on spending most of my time upstairs (as both my bedroom and my workspace are upstairs), and even if I'm downstairs it's hard to see if someone's at the door. I was thinking that I could get a wireless camera set up outside my front door with a live video feed running on my computer, and possibly having it somehow visible downstairs as well. However, I have absolutely no idea how to do that, and other tutorials I've found only take photos, or require an extremely expensive camera. I don't need anything too secure, just a digital peep-hole, essentially. Any ideas and suggestions will be greatly appreciated. 

you could use one of these:
There are lots of similar ones for cheap at amazon, and you could also mount a cheap display to it, so it would be like an wireless doorbell with video surveilance, which are also cheap.
Wireless web cams you just set up the cameras and down load the driver then you can view the camera through my computer as well as save the video on the internet just encase something bad happens.

This is the system I use when dealing with dishonest police and I have never lost a case in court.

People lie video doesn't.
If you have a throw away smart phone (with camera) you could set that up as an IP camera. You would need a power source for it but that shouldn't be too much of an issue. For example you could set an Android smart phone running a free app like IP Webcam and monitor it using it's web gui or directly through VLC.
Wireless cameras are very cheap. If you want to make an IP camera, then you can do that with a raspberry Pi, the Picamera and some assorted bits. You'll probably be out more than 100 bucks doing that.