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

^{n}y=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