A typical 20k whole house generator with transfer switch installed with an existing propane tank in our area is about $10k. You might get away with a smaller generator, but it won't save you much. The labor and transfer switch are still the same.
If you want to be frugal, install a 500 or 1000 gal. cistern at each house. Take turns filling them over a week or so. You can buy a jet-pump from Home Depot that you can connect to your house water supply line. Add a T, and valve accordingly. The jet pump can suck/push water 15-50' depending on the model. You don't need a pressure tank. They run on 120 or 240. If you lose power and big box store 5000W generator will easily be able to run the jet pump. Worst case, is drop a bucket in the cistern and pull the water up manually.
