Effective revenue for producers of a market good is demand * base_price. Supply has no effect on effective money gain. The most important reason of their unproductivity is that there are simply way too many employed people at urban centers: players only look at unemployment values and since urban centers are incredibly profitable per level with regards to input and output they can always hire until there is labor competition.

One of the most important aspects of their mechanic is to create demand for labor and therefore encourage migration. The players choice is either to encourage migration or stay with low wages. But as said, players generally don't look at urban center employment. It is usually much better to treat urban center employees as people waiting to work at industries in my opinion (especially with throughput bonuses).

As I said: revenue is demand * base_price, and if there are considerably less amount of urban center employees and therefore production, the urban center employees have much more productivity, therefore much more wealth / SoL / clout.