If there is a duty of justice to contribute to society, which asks individuals to produce a specific amount of goods and services that can be redistributed, we need a decision-procedure to know when we have done our part. This paper analyses and critically assesses the commonly suggested decision-procedure of relying on market prices to measure the value of one’s contribution. It is usually assumed that a high salary indicates that one’s talents are put to good use, but this presupposes both that market prices of labour are correct reflections of supply and demand, and that market prices are correct reflections of social value. I criticise both assumptions and argue that the social value of a contribution cannot simply be a function of its market value, but is also influenced by the principles of justice that support the duty to contribute. Further, the market solution is incapable of valuing contributions that lack market prices, like non-marketised care labour. The market solution thus fails as a decision-procedure under other than special circumstances. This does not mean, however, that we need to give up the idea of a duty to contribute.