I think the OP needs to bring more specific information about the deal to get our advice. Technically, NOI should = (income - expenses), but not including the debt service. If your NOI, by that standard, is 10% of the purchase price, then it appears to be 10% cap rate which is potentially a very doable deal. But there are other factors that need to be considered and don't forget to do your due diligence to verify that income and expenses are accurate.
With a 10% cap rate its pretty easy to be significantly cash flow positive.
As nationwide said, a fairly good estimate of what expenses ought to be is 40-50% of the income. So basically if they are collecting X dollars in rent, etc, then cut that amount in half to get a rough estimate of the NOI.
If they report expenses lower than that amount then it might be possible to actually be lower, but verify why. The long term expenses are going to be closer to 50%. All sellers will try to present their numbers as favorably as possible. But you also need to know the area. Some geographical areas have higher expenses due to taxes, high maintenence costs, etc..so you need to know if your area tends to be under or over 50%. But for a rough estimate, use 50%.
Also, when doing your due diligence, make sure to have inspections to find out if there is any deferred maintenance, which is even more likely to be the case if they have historically had low expenses. That would mean that they have postponed taking care of certain expenses that should have been addressed. As you find out what those things are, you have to negotiate the sales price down because otherwise you will soon be faced with those expenses which are over and above what they have been reporting for the past couple years as their normal operating expenses.