@Cory O'Dell my answer is stick to your numbers. If everything works out the way you show, then you are getting about 10% cash on cash ROI. I don't get excited over 10% but especially when that's only $42. That being said however the upside would be you at the top of the list for his other properties but again he probably is doing the math just like you. If you are willing to take a 10% ROI on this one he will probably figure you will take 10% on all of them and price each house according to that logic.
Only you know if it's worth it or not and if things go even a little sideways $42/m doesn't leave much room for oops. Personally I don't like it here's why.
Typical Duplex in C neighborhood in Dayton (where I invest) I can get all day for about $70K.
3/1 each side easily rents at $650 = $1300/m
traditional mortgage at 6.5% 30 year fixed payment is about $350/m
20% down payment = $14K with closing costs of about $1K = $15K out of pocket
Factoring all things like cap x, maintenance, vacancy, etc leaves about $715/m (1300x.55 rough numbers)
$715 - mortgage payment ( $350 ) = $365/m cash flow
$365 x 12 months = $4380 divided by $15,000 = 29% TRUE ROI!!!!
The only catch to this is you need 15K out of pocket not 4K. I don't know if you have this now but if not I'd look for this type of deal and a partner before what you suggested...just my $.02