You have factor in fuel, insurance on the vehicle, pension contributions, and all the other crap we all forget about. I used to think the same thing that it would be cheaper to eliminate more OT, but someone else on here pointed out all the extra costs of adding even one route and it kind of made sense.
I know, I've heard it too. Without seeing actual numbers I would just be speculating. It is probably more of a case by case situation, and the ie folks making the call about number of routes may not factor in certain things like drivers in progression, 9-5 list, etc. It may be that fewer routes is just a rule of thumb rather than the best call for every situation.
One thing that I think gets overlooked on the other side of the argument, however, is the long term cost of running fewer drivers for longer hours. Wear and tear on their bodies likely results in higher injury rates, and exhaustion likely leads to higher accident rates. Either of which can create much higher costs than throwing in an extra route or two daily.