I've mentioned that I eat Dannon Light & Fit yogurt pretty often, and I buy it only when it's on sale at $0.50/6oz. I came across instructions for making yogurt in a slow-cooker (one of my favorite appliances) and figured I'd try my hand at it. It was remarkably easy. I used a three-ply aluminum core stainless steel pot to speed things up, but you'll want something that holds on to heat well, not a cheap thin pot. I found other information online (Wikipedia) to corroborate and refine the technique. You will need a decent thermometer.
1 qt of milk at ~$4/gal = ~$1
1 Tbsp yogurt
Heat the milk to 180 F and maintain that temperature for 10-30 minutes. A longer time should result in a thicker yogurt as the heating process makes the proteins play nicely with each other. Let the milk cool to about 115 F, then whisk in 1 Tbsp of yogurt. The goal is to integrate the yogurt "starter" at 113 F, but adding it in will reduce the overall temperature of the mixture. So, if you're adding cold yogurt, maybe do it when the milk is at 117 F, but room-temperature yogurt can be added at 115 F. I don't remember enough from physics to figure it out exactly. There are 64 Tbsps in a qt. Ideally, you will hold the temperature at 100-110 F for the next four hours. I just put the lid on my pot, wrapped it in a towel and put it in my oven. Every couple hours I turned the oven on low (170 F) for twenty seconds. Since I wasn't maintaining the right temperature, I left the pot in for six hours, and some people leave it for 8-10 hours. Then I moved the pot to the fridge.
The yogurt was quite tasty. I had a bowl for breakfast with Grape Nuts cereal and honey. The yogurt will be thicker at the top, and will be much thinner and less sweet than grocery store yogurt overall because most American commercial yogurts add starches, sugars, and gelatin that thicken, preserve and sweeten it. This picture was taken after half the yogurt was eaten. You may see the difference in thickness between the glob of top-yogurt and the surrounding bottom-yogurt.
There are many things you can do with yogurt besides stirring in some honey or fruit jam. It can be used in dips, sauces, and in some drinks and entrees. I will try to find some good dishes to make.
Save a bit of the yogurt you make to be the starter for your next batch. You can make as much as you want in a batch, but keep the 1:64 ratio of starter to milk. The milk turns to yogurt as the bacteria from the starter eat lactose and poop lactic acid, which brings the milk proteins together. The acidity helps protect the yogurt from unwanted bacteria. The yogurt should keep well in your fridge for about 10 days.
Depending on what yogurt and milk you buy and what your price thresholds are, making your own could cut your yogurt expenditures to 1/2-1/6.