im not sure that i am right... but i have always thought of it as a multiplier.
im really not sure what that multiplier is: but lets assume for example that it is 1.10 (10% advantage)
so, if you have 3 HV worth of effort in, it might only be worth 0.3 HV
but if you have 30 HV worth of effort in, it might be worth 3.0 HV
i will say that up until recently i was under the impression it was a very small almost insignificant factor (at least in d3 and d2)
then, in a huge battle in d3 (i think we both spent near or more than $10K on a local recruit) i ended up feeling it was significant,
i won the battle, but had to spend more than i thought i should have.
based off that battle, if this thing is a multiplier, i feel that it is at least 1.10, maybe even as high as 1.20
and, i think that all of your efforts get that multiplier, not just those efforts that occured early on.
this theory would seem to account for the fact that it usually seems insignificant at d3 and even d2, where huge numbers of HVs are not likely, as opposed to d1 where the increased dollars could allow for 50 or 100 or even several hundred HVs.
again, take that all with a grain of salt, i could be dead wrong. it woudnt be the first time.