Do you know of anyone doing this on a large scale?

I probably could've saved at least $10k or so if I did a rental grade product.

On the growth of your operation...Probably much depends on your circumstances and lifestyle..of course, you get economy of scale with multiple units in one structure (one mortgage, one insurance bill, tax bill, one roof, one lawn, one parking lot/driveway, etc), so a fourplex  gives you even more scale than the duplex (or your question may even be about 5 plus units)..

Please tell us more:Is this wall in a basement or above grade?

If the house is slab on grade, you'll have a bigger problem, and if you aren't all that experienced with these types of repairs, it's probably better to not make this deal. 

Say your houses is a slab on grade single story house with a finished floor elevation of 105'.

Not surprising when you think about labor costs vs rental grade material costs.

Alternately, if you have something solid behind it like a stud behind the drywall, you could predrill a couple holes and use a cabinet screw (exterior grade, we use GRK from Home Depot) to gently pull it back against the wall.

If they're older aluminum/contractor grade they probably aren't very energy efficient.  

My site just takes basic bullet points of a deal and classifies them in grades.