Neat concept! I like the idea of having a low-performance but low-complexity alternative to some of the more exotic propulsion concepts shown so far. I could see NHSER or something like it being really useful in the early stages of a colony, as it gives some (limited) launch capability without having to deal with the cost/complexity of building or flying in a full fuel production setup.
I think OP's use of "running costs" more generally refers to the cost and complexity of fueling the engine. For example, the NHSER concept here has a low running cost since it just needs water to be mined out of the regolith, a chemical engine would have a middling running cost due to the need to further process the raw materials into a usable state, and something like an antimatter engine would have a high running cost due to the high complexity and energy requirements of generating antimatter.
