What's the cost basis when converting OO to rental?

[John Ford]

I recently converted my previously owner-occupied home to a rental property. But I don't know what numbers to use to analyze how the house is performing. 

I paid 189,900 for the house in 2005 (about 192,000 including closing costs, etc...)
I currently owe about $152,000 on the house
The house would appraise at about $350,000 based on recent comps

So if I wanted to analyze the performance as a rental...cap rate, for instance, what do I use as the cost? Is it what I still owe, my equity, the fair market value, something else?