It is based on a percentage of one's salary. A low-income earner will contribute a smaller amount, but will still receive enough to replace his salary. If a worker is already below the poverty threshold, that will not change. There is also a social aspect to this issue, which can be covered by a government contribution, if it wishes to subsidize very low-income earners.
But I am referring to a plan which both employers and employees would contribute to, and which would replace plans which, in my view, are more expensive.