Walmart is usually around $7.xx for 2.5 gallons of their brand.
DEF is all the same, there’s no secret sauce that makes one brand differ from another.
Absolutely, Walmart has actually dropped it below seven dollars in our area. It’s the freshest and meets all specifications. No reason to pay more anywhere else. Used it for years in two or three different diesel vehicles...
Heat is the main enemy of DEF. In 70° or below it can last for 18 months, 2 years no problem.... but in continual 90° plus, it’s life can be shortened to as low as six months. It just slowly degrades....
The way I roll with DEF is I let it go down until the first reminder shows to refill it, then I just go and dump a 2.5 gallon container in, and wait for the next reminder. That keeps the whole tank pretty fresh and able to withstand any conditions and lets me motor quite a few thousand miles down the road before I hear from the indicator again....