I asked Walmart and they told me that the 5mg warfarin is the most prescribed strength and they buy in massive quantities.
The other thing with Walmart is you have to really watch it if you are buying generic meds that have been out for a long time. For example, I can buy 300 lasix tabs from Drugstore.com for $17 but Walmart sells 30 tabs for $4. Obviously, they are not cheaper on some drugs.
Also, the $4 is for 30 tabs in most cases even if your doctor prescribes more than one tablet/day. I take 2 potassium tabs per day so I have to pay $8 at Walmart for a 30 day supply. Still a good buy but not really clear when they say they charge $4 for a 30 day supply. You have to look at their list to find out how many pills you can get for your $4.