It never ceases to amaze me the ridiculous rules and laws that make us spend more money than necessary and prescription drugs are at the top of the list.
Apparently pharmacists (through contracts) adhere to a gag order in which they are not permitted to disclose to a patient that a prescription might be cheaper to purchase without using insurance. This is in place to pay benefits managers (clawbacks) who negotiate co-pay amounts. But the pharmacist can divulge this information if you ask.
Here's an example of how it works according to Bloomberg.com:
Insurance co-pay $15.00
Pharmacist receives $5.17 plus $2.05 for the cost of the drug
Pharmacy Benefits Manager receives $7.78
Without having to pay the benefits manager (included in the co-pay), the drug would be cheaper.
There are also available prescription drug cards that profess to offer prescriptions at a discount rate. These may not always be cheaper either.
Next time you go to have a prescription filled ask the pharmacist if you can purchase it for less by paying in cash without using insurance. Don't assume it will be cheaper by paying a co-pay.