Consumers who have a prescription drug benefit might not care what their medication actually costs, since they only pay a very low flat co-pay. And a new survey shows drug companies and drug retailers may be trying to exploit that fact.

A survey conducted by Consumer Reports found that not only do prices vary from store to store for the same drug, but the fluctuations can be dramatic -- sometimes more than $100 for the same prescription even with the same chain, depending on whether consumers are filling their prescriptions in, say, Omaha, Nebraska, or Billings, Montana.

Consumer Reports said it placed more than 500 calls to 163 pharmacies nationwide to gauge price differences among four prescription drugs, three name brand medicines and one generic.

For a three-month supply of pills for the urinary incontinence drug Detrol, for example, the price ranged from $365 to $551.

CR also found significant price disparities for the two other name-brand drugs it studied: for Plavix (which prevents blood clots), the spread was $382-$541, and for Levoxyl (for treating hypothyroidism), prices ran from $29 to $85. And for the generic alendronate (for osteoporosis) the price range was $124 to $306.

In the small scale market-basket study, Costco was the cheapest for the four drugs CR sought quotes for, followed by and Wal-Mart. Walgreens and Rite-Aid were among the priciest for the four drugs.

Besides calling different stores and comparison shopping, the magazine offered other cost-saving tips:

• Don't rule out independents: Though they're not the cheapest overall, many mom-and-pop pharmacies are highly competitive and offer top notch service.

• Talk to your employer: Benefits administrators can provide details about pharmacy benefit managers, also known as PBMs.

• Buy generics: They can cost 20 to 50 percent less than their brand name equivalents.

• See if there's a discount program: Some stores have programs for those 50 and older; other programs are open to anyone without insurance.

The Consumer Reports National Research Center surveyed 40,133 readers to find out about their experiences at drugstores. One striking finding: readers sought pharmacists' advice about prescription drugs at just 38 percent of walk-in visits during the course of a year. That's down from 50 percent since CR's last survey in 2002.

"That's a pretty significant shift in the consumer-pharmacist relationship," said Tod Marks, senior editor at Consumer Reports.

The survey also found that only 33 percent of prescription drug buys were mostly or entirely covered by insurance coverage. In 2002, the last time CR surveyed readers about their drugstore experiences, 65% of prescription drug buys were covered. "More people are digging deeper and paying a larger share of out-of-pocket costs for prescriptions drugs," Marks said.