Academic studies (see the works of Laurie Williams) suggest that, on average, a task requiring 2 hours for a lone programmer will require 1.25 hours elapsed for a pair. So there is an apparent increase of 25% in effort expended.

However, there are also two compensating gains. First, the task is completed sooner, which speeds up the overall feedback/learning loop. And second, the pair will generally produce code that's more correct and easier to maintain, effects which dramatically reduce the code's long-term cost of ownership.

In summary, pair programming is cheaper, better and faster in almost all cases.