It's not necessarily the farmers who are the price problems. I looked into opening a verduleria a year or so ago and found that the farmers sell for very low prices, the guys who buy from the farmers bring the produce to places like Mercado Central to sell, and then the verduleros go to Mercado Central and buy from them (Mercado Central delivers to verdulerias). Quite a bit of markup on each leg.
On top of that, as an aside, I found out that the verduleros (at least some of them) make more money by selling to restaurants in the neighborhood - they markup the price significantly higher for restaurants than they do for individuals they are selling to our of their verdulerias.
The verduleros that make the most profit have their own trucks and have made deals with farmers directly. But instead of charging a lower price and getting a lot of business (competition = evil!), they all pretty much have the same retail prices, at least within neighborhoods...
As far as incentive for the farmers - that's a good question!