The cost savings are long term with lower service growth, not reducing costs today, as the city is still growing. The planning cost savings are costs not paid by taxes, but are instead costs paid by people who buy new units.
Any attempt to quantify that dollar savings on services (possible) would only be relative, not absolute. It would be possible to project that out over time. Turning that into levy differences per typical residential is damn near impossible beyond the immediate year, to the point where I would call any long term projections like that fantasy.
People also have a nasty habit of calling projections promises, and interpreting a projection of a reduction in levy from where it would otherwise be as an absolute cut, and then would feel even more mad when the 'promise' of an absolute cut was 'broken'.
Frankly, I think people who would be converted by the economic efficiency angle are already converted from the 'supply and demand works' and 'less government rules' angle.
It is best to recognize opponents of most density as a vocal minority, and those who are further down the engagement spectrum (those who show up) as the extreme representation of that minority (not that they're extreme, but that they're further down the engagement spectrum). It is best to be cognizant that attendees at planning meetings are not representative of communities for the most part, in age, in background, in income, and in home tenure status, and for Council to internalize that so they can project perhaps what the overall opinion might be, given who shows up, before they decide their vote.