Michelle and Robert constructed a wooden bridge for ATVs over a deep creek on the edge of their property. To recuperate the cost of the materials, they decided to charge an annual toll of $8 to each of the 120 members of a local club. A survey showed that for every $1 the toll is increased, 4 members wouldn't use the bridge anymore. What is the best toll charge to allow them to recuperate the cost of the materials the fastest?