Cap Rate.. based on previous owners purchase

[Tyler Hallman]

I am new to running commercial deals and when looking at cap rates and NOI the numbers don't quite make sense.

Example: 

Asking price - $1.175m

Cap - 6.36%

NOI - $74,000

Its a 5 unit in Denver, Colorado. I can see how their NOI is $74,000 if they bought this 20 years ago for 500k and their monthly payment for the entire complex payment is around $2500 per month but now they are making $1800 per unit per month. So for example as the purchaser (me) my new monthly payment for that entire unity is say $5000 per month. That very roughly makes the NOI 48k dropping the value by a lot. Am I doing these calculations right or thinking of this the right way? It seems pointless to advertise the cap as it sits because at the new purchase price that will change the NOI drastically and in doing so changing the cap rate. Thank you for all your help!