An Apprentice in Imperialism 2 is the second-to-bottom type of worker, with productivity 4.

He needs a luxury to maintain that productivity: refined sugar.

Training a peasant to become an apprentice requires the appropriate technology, Apprentice Workers, and $100 and 1 piece of paper.

Training an apprentice to become a journeyman (having productivity 6) requires the appropriate technology, Trained Journeymen, and $500 and 5 pieces of paper.

The step from 4 up to 6 is actually of more value than the step from 1 up to 4, because the apprentice starts using luxuries, which require two labor units per turn out of the three "gained", whereas moving up to become a journeyman simply switches to a different luxury.