I would say the cost goes way beyond that... the cost of doubling your time to market can be brutal. Not to mention extra strain on other resources (including your own time) having someone consistently missing deadlines.
My current company sometimes misses this issue. The developer you've outsourced to can take 1.5x but if that slows down not just his items but dependent work, the cost can be very difficult to determine.
The bottom line is that outsourcing looks great on paper but then just murders you with hidden costs. Delays to market, code maintainability, number of bugs, etc all go the wrong way once you outsource, but none of that is quantified on any C-level report.