My only advice is to test it and find out. If you run for several months with a positive RIO, then all is well. If not, adjust the campaign or go back to the status quo.
One other comment on this: I have seen people do these types of tests in the past and find what they *think* is a positive ROI from their PPC campaign. However, they had ignored the cost of cannibalization of the 'free' traffic. Depending on the keywords and industry, the cannibalization of traffic from the natural listing is going to vary - but just be sure to evaluate the effect the campaign has on overall ROI and do not evaluate the natural or PPC traffic in isolation.