Costco is already giving big box retailers a run for their money. Now it looks like the popular wholesaler is destroying the competition in the generic drug market.

A new study by Consumer Reports found shoppers could save more than $100 per month by shopping for generic drugs at Costco over major drugstore chains like CVS.

In their investigation, secret shoppers called up 200 pharmacies throughout the U.S. to get non-insurance prices on a month’s supply of five blockbuster drugs that recently went generic: Actos (pioglitazone), for diabetes; Lexapro (escitalopram), an antidepressant; Lipitor (atorvastatin), for high cholesterol; Plavi (clopidogrel), a blood thinner; and Singulair (montelukast), for asthma.

The differences were pretty astounding –– as much as $749, or 447 percent, between the highest- and lowest-priced stores. The secret shoppers phoned in a range of pharmacies, including Walgreens and Rite Aid, but CVS was clearly the priciest. And Costco beat them every time.

Lexapro: $7 at Costco vs $126 at CVS.

Lipitor: $17 at Costco vs. $150 at CVS.

Plavix: $15 at Costco vs. $180 at CVS.

Like other big box retailers, Costco gets away with this thanks in part to its massive inventory of goods. It won't tank simply because it gives a discount on generic drugs, whereas pharmacies rely heavily on their prescription sales.

The Takeaway

This level of price disparity is shocking enough on a one-time basis. But some of these are drugs that people are prescribed for a number of years –– and in some cases, for a lifetime.

It's important for consumers to remember their bargaining power at the pharmacy. Independent pharmacies are more likely to negotiate lower price points to beat large competitors. And if they won't budge, then simply take your business elsewhere.

“A consumer can’t assume that the price of their prescription medications is set in stone," said Consumer Reports editor Lisa Gill. "You have to ask for the best price and see if your pharmacist will work with you."

Even simple steps like shopping outside of large, populated cities can yield cheaper price points. Consumer Reports found a 30-day supply of generic Actos at a pharmacy in Raleigh, N.C., for $203, vs. just $37 at a pharmacy outside of town.