Duplex Analysis - What's wrong here?

[Spenser Harding]

Hey BP, came accross a duplex and I feel the numbers are out of whack some where so I need some help as I don't want to over pay.

-2 studio units @ $500/month, rentometer/market has average rent around $575.
-Listed for $99900
-Total Monthly Expenses $280 (water/sewer/garbage + monthly insurance and tax, $3360 annually)
-NOI of $720 monthly ($8640 annually)
-Cap rate 9.70
-GRM of 30

I don't get how they got 30 for GRM and when I calculate cap rate, 8640/99900 = 8.64, not 9.70. Max purchase price I calculate using the stated rate is 9.7*8640 = $83808. The listed numbers don't add up so I need some other oppinions!