Unfortunately most of the expensive stuff is in the loco!
But you only need it once per train... but it also has to be bigger...
In terms of overall cost, the critical length is apparently five carriages. Above that, loco-hauled is cheaper. Below that, a DMU is cheaper. But cost isn't everything; if platform lengths are limited and passenger numbers high - for appropriate values of 'limited' and 'high' - then the loss of passenger-carrying space to a locomotive is unacceptable.
Of course,
then you get into the question of whether the savings from running DMUs are more or less than those from running loco & stock and forking out to extend the platforms. Ususally extending the platforms will be expensive, because of the impact on the track and signalling. But sometimes not.
Marketing could fall either way - the loco-hauled stock
might offer a better cabin environment, the DMU
might offer faster journey times, either factor potentially winning or losing business overall.
Really, to find places where loco-hauled stock is appropriate, you need a service which has enough passengers to fill a long train, but not one so long that it's getting close to length limits. I'm not sure how many services meet those requirements.
To be honest, I suspect that a lot of the longing for loco-hauled trains is actually longing for the big trains that were hauled by locomotives. Whether they were locomotive hauled, self-powered multiple units, or cable-hauled to the top of a hill and let roll down isn't really the factor. The fact that they had restaurant cars, buffet cars, van space for parcels and luggage, compartments, and neatly-uniformed staff is the point.