I haven't seen the one at Walmart, but I know Harbor Freight sells an OBD-2 code reader for $40-$50.
One thing I'll point out. The cheapest code readers typically only support generic OBD2 codes. They generally don't support OBD-2 enhanced or manufacturer specific codes.
Another point to make. The majority of the time, a DTC by itself isn't enough to "tell you what is wrong with your check engine light." A DTC is good for telling you where to begin your diagnosis, but can't do the diagnosis for you.
Of course, those kinds of functionality can be had, but at greater cost. If you do decide to go for the cheapest code reader available, recognize that you will be limited in how much diagnosis you can do, and you may still be dependent on someone else to get a good diagnosis.