Which would equate to about 45-50k profit, which would be awesome!

Hopefully you are working with a good agent who is well aware of the timelines if they are specified in your contract.Doing the simple math of acquisition and selling costs, hard money charges....those numbers equate to a $27,000 profit before accounting for things like marketing costs to locate and acquire the poprert.  

I quantify the results and extract equations from the results to faithfully, and predictably reproduce said results.Having said this,  the system works.

I found out they owe 4 years worth of back taxes equating over $21k.  

I wouldn’t let “cheap” enter the equation.

Is there an equation to calculate the water versus square feet for each unit? 

banks will typically allow loan amounts 80% LTV minus mortgage balance, so depending on what that amount equates to for you, you can take it out at one time with a fixed rate and term in the form if a home equity loan, or over time as a HELOC (typically variable rate, shorter term, but sometimes only requiring interest only payments on the amount drawn). if you are renting this property, the rent payments can pay the second loan note during the seasoning period until you can cash out refi into a new mortgage (using the cash to pay off your equity loan/line)...then rinse and repeat!

One thing to remember to factor into the equation is the refinancing loan costs and the new higher debit service.

Higher price does not always equate to higher demand. 

If you have a 100 unit apartment building with all 1 bedroom units and comparable floorplans and units are renting for $800 and your building is renting for $600, that would equate to a $200 loss to lease.