How to tell if a deal is good with owner financing

[Kyle Meyers]

I recently read this blog post and it brought up some new questions for me.
http://www.biggerpockets.com/renewsblog/2011/07/21/using-seller-financing-to-acquire-properties-still-works/

The concept of the post is that a more expensive purchase price can still be a good deal with better loan terms. I understand the idea, but how do you measure if you are getting a good deal then?

On a standard purchase you would evaluate the property using the NOI and cap rate, but that factors in the purchase price. If you are making the deal better by getting good terms on owner financing the cap rate won't show that.

I am guessing the way to measure it is with the 50% rule and cash flow. So you would figure out the market rent and then 50% would be the estimated before debt service cash flow, then you would subtract the monthly payment to get the cash flow.

What would be a good cash flow? $100/month? $200?

Is that even the right way to calculate how good the deal is?