Some years ago, BR did some analysis of the 'whole life' (purchase+fuel+maintenance) costs of DMUs versus trains with the power equipment concentrated in one or two vehicles (loco hauled/HST). I think the crossover point was at about 7-8 coaches in length - so a full length HST was cheaper per seat than an equivalent length DMU, but shorter length versions were more expensive. So Regional Railways longer distance trains went DMU, and InterCity stayed HST and loco-hauled.
To compare short HST versus DMU running costs you'd have to measure both on exactly the same usage patterns (services/stopping patterns/speeds), otherwise you're comparing apples and oranges. Remember the two HST power cars weigh 140 tonnes alone (which you're dragging up the hills and accelerating away from stops and speed restrictions) - for a short 4 passenger-car HST that's 35 tonnes 'overhead' per passenger car (making each car weigh the equivalent of 70 tonnes, for comparison with a typical 23m DMU car of about 40 - 50 tonnes).