From what I have read in the past, there are different models that different coops follow. Most have a limit on the number of dwellings, both from the government and coop planning, if they have hit the max number you would have to purchase from an existing member. From the coop properties I've looked at, they can be well below market rate but that's because you are buying into a community and there are expectations and responsibilities not found in non-coop living. A little like American HOAs.

There are some coops that have strict requirements to join, like womens only coops or some that follow certain beliefs(environmental, political or spiritual and some can feel a bit cult-like). You can't demand market rates when you have such heavy restrictions and demands of property owners.