Like so many things. It's just better homemade.
The savings of course depends on whether or not you buy your milk. But regardless there is still the control over the quality.
My favorite for soaking chicken to fry, of course in biscuits and anything chocolate that calls for milk. Besides just drinking it!
Buttermilk can be re-cultured from the first starter batch and continuously after that. We purchased our starter enzymes from Cultures for Health. Their site is easy to follow and full of tips and recipes. The starter batch is simple. (screen shot from their page)
After the starter batch add 1/4 cup of your reserve to 1 Quart milk and let sit in a warm spot 70-77* for 24-48 hours. (Full explanation is linked under re-culture, above)
(You will need a thermometer and a crock pot)
I use a lot of yogurt. I started out replacing a little at a time of mayo or sour cream in recipes like pasta salad and topping for Mexican foods. Yogurt can also replace oil in a lot of baking items. A splash of olive oil will give it that oily texture that is sometimes needed.
Like buttermilk, yogurt can be simply re-made from your starter batch. 1 Quart of milk heated to 160* and cooled to 110*. Then add 2-3 T of starter yogurt and allowed to incubate for 5-8 hours. You can buy the yogurt makers that are set to the right temp etc. I was given one and am trying it next time.We have been using a crock pot and have had very good luck with. Heat the crock pot with water in it until you are ready to let the yogurt set. Dump the water, add the yogurt milk mixture, be sure to shut it off and unplug. Wrap the whole thing with a heavy towel to hold in the heat.
Both can also be started from store bought. Make sure to get plain yogurt with live cultures. The drawback to this is that the store cultures are not as "alive" and after a batch or two they don't really work anymore.
When making your own you will see that the textures are slightly different from store bought. The buttermilk is a little thicker but the flavor is amazing. A good stir with a spoon or whisk will loosen it up. The yogurt is slightly more watery but again a stir will bring it together. Also letting drain though a strainer or hanging from a cheese cloth will tighten it up to whatever texture you like.
At our local store a 32 oz (4 cups) container of yogurt is about $4. A gallon of milk is also about $4. I can make 4x as much yogurt for the same price.