Thanks for that article!! We're about to start getting raw milk, and I'm hoping to make some cheeses, etc., and perhaps my own butter from it. (I promise, I'm not quite the hippie that I sound like, I just think this stuff is neat.)
I feed as much as 60% of the meal as tripe. I buy tripe blends form Texas Tripe. He's actually who I buy a lot of stuff from. He does a number of blends that include tripe-- and they make the tripe slightly less stinky if mixed with something else. I think I maybe started out with 10-20% of the total meal to start with...? I can't remember. Mine get such a wide variety of foods that they tend to handle big, sudden changes well. Not sure where any of you are located, but if Texas/Oklahoma/Western Arkansas, Texas Tripe has been a fantastic resource.
One thing I did to keep costs down was to contact the local farmers directly by email, and then I visit the farmer's market with a dog in tow.

I explained to them what I do, what kind of things I'm looking for (mostly offal, poultry bone, etc.) and what the benefits are of doing this (it's not just great for the dogs, it uses up items that people tend to be squeamish about or things that are inedible for us and that therefore might otherwise go to waste). It appeals to many of our own senses of economy and conservation, and it's a win-win, since it becomes another revenue stream for them and a quality local source of food for us. I know a few farmers have started having the excess chicken ground and sold at our farmers' market for pet food since I started asking around. I can get a grind of frames, necks, feet, liver, heart for $2.25/lb bulk price-- all free range, etc. Lots of meat on them. I got the whole ground chickens from him this week for $2. I can also buy directly from another facility a bit of a drive away and get more AB/hormone free, free range frames (unground) for about 20 cents a lb. A little less meat, but still great. And I'm supposed to go pick up some pork liver and beef tongue from a small operation that needs to clear out its freezer.
I also visit our local abattoir. They process most of the local small-scale farmers' livestock, as well as that of hunters. They are Animal Welfare approved. They sell a lot of organ meats to me, but if I call ahead of time, I can get them to save sets of lungs, heart, kidney, liver all still attached (unprocessed) for $10-- I put in the elbow grease on those. Most everything they do is AB/Hormone free-- a mix of grain and grass fed.
And then I buy cases of a few feed lot/industrial items-- chicken hearts and gizzards by the case, pork hearts-- all around $1/lb. I'm always looking for better deals on higher quality products, but the way I look at it, even if I'm feeding them stuff from the grocery store, it's better than most kibble, especially the ones we have who have special dietary needs. I have 4 medium-large dogs, so I can only afford to do so much.