As far as i know, a typical coop will require members to deposit "social stake" of an arbitrary amount of money (and it doesn't have to be large, the stake for the cooperative "bank" i am a member of is 5$, for exemple). In a worker's coop, I would expect new employee to have to buy their membership right away, or to have it deduced from their first (few) paychecks. In an employee's coop, membership require being an employee; when someone leaves their employement, their stake is given back and their voting privilege rescinded.

The key in all of that, is that the cost of membership isn't tied to any sort "market valuation" for the cooperative, because the cooperative ownership isn't part of any market.