If you work on your Commercial from the right seat, it will probably save
you some money on your CFI. If you can do most of the flying in a
inexpensive fixed gear and then do the required maneuvers in a complex, that
also may save you money. But, most of the CFI work will be
ground/desk/classroom/lesson plan and teaching work, not flying. Much of
this work will be done by yourself or practicing on willing victims, empty
chairs, stuffed animals and or blow up dolls, so this will not be expensive.
People used to see and hear me taking to myself all the time. Most of my
expense was incurred in building my CFI library, buying teaching supplies,
and practice teaching to my CFI, which I fully expected and happily paid him
to do. I borrowed other CFIs to run my techniques by them over a beverage
or dinner. Once I was up to speed I took the FOI, AGI, and IGI and started
to help teach ground school. Then I developed from scratch my own
Instrument ground school program and syllabus. This was invaluable.

In short, you won't spend as much on airplane rental as you do for your
commercial, but you'll spend more on books, supplies, and ground
instruction.

Ballpark? hmm $1500-$2000? just a guess.

Jim