Is the durability of more expensive chains better than cheaper ones? Do expensive chains last longer? I get 300-400 miles using Wally World's $10 8 speed chains while riding off road or dirt.  I monitor and replace the chain at close to 1 percent stretch. Would a 'better' (i.e. more expensive) chain last longer? Has anyone done a comparison of chain life and longevity vs price?
No. The only advantage of expensive chains within a specific brand is the minor reduction in weight. The ones with extra plating do look nicer and provide some corrosion resistance, but it takes almost no effort to keep your chain rust free. 

There are differences between various manufacturers, but in general those relate to shifting and how the quicklinks work, not the chain life. (i.e. if you like how SRAM chains work on your bike, buy the cheapest SRAM chain. )

300/400 miles is a very short life span for a chain. Two things might help you extend the life of your chain. 


If your rear cluster is worn, that will quickly wear out a chain. 
On the other hand if you replace your chains regularly a rear cluster can last a very long time. If you have lot's of miles on the rear cluster consider a replacement. Any time you replace the rear cluster you should replace the chain. Front chainrings can also cause wear, but they take a lot longer to wear out. 
Excess lube on the chain. In a dusty dirt environment any lube on the outside of the chain creates a grinding paste that will wear a chain quickly. Chains actually move very little. All they really need is enough
lubrication to prevent corrosion inside the rollers. If you're riding in
dirt and dust wipe the outside of the chain as clean as possible before installing it. 


Point 2 is controversial, all I can say is I ride on fine clay soils that destroy chains and it has worked for me to greatly extend the life of both chains and expensive gear clusters.