The specific processes by which miners were compensated for their work and how they fed themselves differed by geographical region and time period within the middle ages, but I will try to give a general sense of how mining worked in the High to Late Middle ages (1100-1500).

Mines were almost never owned by the miners themselves, but instead by the crown or the noble landlord who claimed dominion over the land the mine was located in. The miners had to pay for the right to work the mines. In the case of valuable metals such as gold and silver, tribute was paid directly to the crown if the crown exercised their "regalian rights", or right to claim ownership of property. The payment made was frequently in the neighborhood of one tenth of the ore mined. For mines which produced less valuable products such as iron or coal, payment was often made to the private landlord who owned the land where the mine was located.

Payment to the private landlord could be made in any number of ways, often in a similar manner as payments to the crown, with some of the ore being given to the landlord. The miners themselves made their money by selling the remaining ore in their possession. As for feeding themselves, miners and their families often raised their own food. Frequently in cases across Medieval Europe, the payment to landlords resulted in the right for miners to own land around the mine for the purpose of building their cottages, mills, forges, as well as land for crop and pasture use. It also often included access to streams for forge and mill use, as well as the right to cut down the necessary lumber for their work.

Thus miners were often self sufficient in supporting themselves, and gained their wealth through the ore that was not taken as payment by nobility.

Source: Nef, J. Mining and Metallurgy in Medieval Civilisation. The Cambridge Economic History Of Europe From The Decline Of The Roman Empire, 691-761. http://dx.doi.org/10.1017/chol9780521087094.012