I'm a farmer without employer-sponsored health insurance and so I pay the cash price for my meds (although I just recently acquired insurance via Obamacare). The cost of azathioprine / imuran for me is $29 for 90 pills at Walmart. Another low-cost source (as mentioned above) is Costco, and you don't need a membership to their warehouse club to use their pharmacy.
To get the lowest price at a pharmacy (if you are uninsured) you have to ask for their "lowest discounted cash price" and you may have to talk to a manager in person at the pharmacy to get that price. If you call a pharmacy on the phone for a price quote, they will most likely give you their undiscounted "list" price. Prescriptions (in the USA) are not priced like normal goods and services. Instead they are priced like other medical services (such as doctor and hospital charges) which are priced at an inflated list price to allow room for the large discount off the list price that insurance companies have negotiated with medical service providers. These discounts are typically in the range of 60-70% off the list price which means that the list price is typically inflated to about 3 times the real/fair/honest price.