Coming back to this thread to report back on the powdered milk experiment! I've been making yogurt with dried milk - I used Nestle Nido full fat milk powder, which is about £3.75 for a 400g tin; I think skimmed milk powder is cheaper, but I like full fat dairy products. 1 can makes about 13 cups of milk, and 2 cups milk yields 1.5 cups yogurt and 0.5c whey, which I mix into bread/pizza dough.
I've found you do need to treat the reconstituted milk in the same way as if you were making yogurt with fresh milk - i.e. heat it to 82 degrees then let it cool to around 46, mix it with a fresh yogurt starter, then transfer it to a thermos/Easiyo container to let it set overnight. Theoretically you shouldn't need to heat it because there's not the same need to kill off any bugs with dried milk as there is with fresh, but when I tried just heating it to 46 it didn't set.
You do need a small amount of live yogurt as a starter, as the Easiyo sachets didn't seem to work as a dried starter, as well as drinking water both to reconstitute and to boil to warm your thermos. I'm really pleased with the Easiyo kit on the whole, as you can make yogurt completely from scratch in it, you don't need to use the sachets unless you want to, although they're a good fallback to stash in the cupboard.
At current prices, both fresh milk and yogurt are very cheap, so I don't think this is a cost-saving solution, but we don't know how prices or availability may change in the future, so I'm glad to have this as an option if it becomes necessary. It also means less plastic going in the recycling bin, which is cool.
Hope this helps @BelliniSurge!