Our team consisted of a junior and a senior developer.
The senior one, apart from doing his own job on this project, reviewed pieces of code that the junior created. This was a benefit for our client, as the result they got was of high quality.
The junior developer costed them $5 per hour less than the senior.
What's even cooler, the junior developer had been raising qualification during the project and became able to solve more complicated problems as the collaboration went on.