It is very complicated when saying "ability to pay". Many aspects will be involved into consideration (majorly on company's finance/accounting situation). There are cases where people who work even for top 100 big companies are still receiving RFE asking them to provide more evidence to prove ability to pay, while people who are in very small firm can overpass this issue without any obstacle. The basic rule is if you were not paid upto the prevailing wage in a certain year, but the net income of your company in that particular year is equal or more than the diffrence between your salary and prevailing wage, you won't have big problem. There are other measures to calculate, but unfortunately I am not an expert on that.
You can go to www.immigration.com, where you can find "forum" in which there is a specific thread focusing on this issue.