The more visitors per day (robots only) you buy, the lower the cost of the visit:
0-99visits: 0.005 USD 100-499visits: 0.0045 USD 500-999visits: 0.004 USD 1000-4999visits: 0.0035 USD 5000-9999visits: 0.003 USD 10000-19999visits: 0.0025 USD 20000-29999visits: 0.002 USD 30000-39999visits: 0.0015 USD 40000-...visits: 0.001 USD