I would gladly pay for 5 hours of training for a combustion endorsement, if I could avoid fuel costs for 40+ hours of training to get my private certificate.
Fuel for a typical 4-cyl trainer will be around 8 gph at < $5/gal. 40 * 8 * 5 is $1600. 5 extra hours of dual in a rented piston plane is going to run you more than half of that possible savings. The net savings will be several hundred dollars, pretty much a drop in the bucket for flying expenses.
Why would it have to be "extra"? ISTM they could just train as they do now, except that they would concentrate all the ICE issues in the five hours in which they're flying with ICE.
Every new airplane type takes some type-specific instruction. True, it could count towards the FAA-minimum 40 hours, but almost no one takes a checkride with 40.0 hours in their logbook. I was a pretty dedicated student and at least a fair stick, and I was still in the 55-57 hour range when I took the checkride. Most people training at towered fields are in the 60-75 hour range at the ride.
Fuel isn't the dominant cost in flight training. (Source: I bought my first airplane during flight training and have been flying for almost 2 decades now. Fuel is more than a rounding error, but tends to run around 1/4 of my all-in costs.)