Just updating. It works exactly as I suspected it would.
My lot value when bills were due was §19,163.
With 4 frugal sims the bill was §121.
It should have been §383 with no frugal sims.
The game calculates bills as shown in
this post.
So the formula is
y=
x×0.75
ny=final bill cost
x=bill cost according to
this postn=number of frugal sims in your household
Not sure if my mathematical notation up there is 100% correct, but it works.
Enjoy
Edit: I decided to edit to add this easy list. (All percentages are rounded)
1 frugal sim=25% discount
2 frugal sims=44% discount
3 frugal sims=58% discount
4 frugal sims=68% discount
5 frugal sims=76% discount
6 frugal sims=82% discount
7 frugal sims=87% discount
8 frugal sims=90% discount