We are hoping to have a decent portfolio built in the next 5-10 years to help supplement our income and hopefully give us some more free time from our 9-5 for when we decide to start a family. 

But to ask a generic price per unit that does not take into account the specific market is like throwing a dart at an invisible dartboard.

We have even gotten to the point where our townhome development is offering new buyers a payment supplement program for 24 months just to drive sales.

I've heard mixed reviews about just grabbing a generic one off the internet - doesn't seem very safe to me.

Could they find a third to supplement the rent?

You used the phrase "replace our income" vs. supplement, so I am taking this to mean you intend to quit your W-2 jobs and go full time real estate once you get to the end of your plan.I am sure you realize that $10,000 / month from rents isn't like $10,000 pay check. 

I was prepared for the new property tax amount as a percentage of the sale price but I wasn't prepared for the supplemental tax invoice which is separate from escrow and I still don't fully understand how it is calculated. 

There is no generic answer.

Figure out a way to supplement your real estate business until you can start earning more.

Definitely helps you stand out from the generic templates.