It's obviously not easy to get figures off the Internet but it looks like an engine might be about 10% of the cost of the plane. A larger engine might be 40% more. So with maintenance, you might save 12% (taking the times 2 guideline above). From that you need to subtract extra fuel to carry the stand-by engine. And then you have to buy the engine, which needs to be much cheaper.
So the question is can you configure an engine to make major savings?