To go with the many unscientific formulas used in the article, here is one more:
consultant_cost = employee_cost + revenue_to_consultant_firm
employee_cost = consultant_cost - revenue_to_consultant_firm
In short: If you actually need a full time developer (and are able to find a qualified one), you are better off hiring him yourself, if you need him for a while.
The article is well-written but it's marketing mumbo jumbo for hiring consultants.
Disclaimer: I work as a consultant, but I'm hired because they don't need my position full time and they don't have other people in house who have the same knowledge.