Co-pay amounts are set by your insurance carrier, and each carrier offers many plans. The simplest explanation would be tiering; medications are categorized in three tiers. Most insurance plans assign a different co-pay amount with each tier.
For example, with a popular plan from a local carrier, there is a $10 co-pay for Tier 1 drugs, $30 for Tier 2, and $50 for Tier 3; while a well-known Medicare D plan sets its co-pays at $0 for Tier 1, $3 for Tier 2, and $7 for Tier 3. Most insurance plans require that the pharmacy collect co-payment at the time of service. Call the pharmacy to make arrangements that work best for you.