Each one completed successfully reduces the mash price by $20 for 48 minutes (1 in-game day) failing the main objective but completing the secondary objective (e.g.
The price can be reduced to as low as $10 by completing bootlegger missions (whether from Maggie or encountered in free roam) or Moonshiner story missions. Once you complete the second story mission, this reduces permanently to $50.
Somewhat inspired by the Bounty Hunter tips post, I've done some research for Moonshiner to figure out just how the payouts and such for Moonshiner work.