A gainfully employed prisoner is liable for the cost of his board in the jail as fixed by the sheriff. If necessarily absent from jail at a meal time, he shall at his request be furnished with an adequate nourishing lunch to carry to work. The sheriff or friend of the court shall charge his account, if he has one, for such board. If the prisoner is gainfully self-employed, he shall pay the sheriff for such board, in default of which his employment privilege is automatically forfeited. If the jail food is furnished directly by the county, the sheriff shall account for and pay the board payments to the county treasurer. The board of supervisors by ordinance may provide that the county furnish or pay for the transportation of employed prisoners to and from their place of employment.