What is the difference between more expensive cocoa powder and regular one?
Just as with other agricultural products, there are different grades and qualities of cocoa beans, and the resulting product is also affected by handling. Valrhona does use very high quality sources and has high standards for processing, but it isn't the only high quality chocolate producer out there, so you may want to give a few other brands a try and compare. There are also different methods of processing the cocoa powder that affect how it works as a drinking chocolate or in different recipes.

On the topic of taste, Serious Eats did a taste test of bar chocolates and found Trader Joe's compared favorably with Valrhona (although they gave the prize to Valrhona). But the comments about the flavors of others in their top 10 may also give you some ideas as to other chocolates to try. They also did a taste-test for drinking chocolates. They were specifically looking at things marketed as drinking chocolate and Valrhona seems not to have been compared here, but again, the notes about the different chocolates might be of interest.

King Arthur Flour also has several articles about cocoa powders and what type works well in what sort of recipe. Of course, they want you to buy the ones they sell, but it's still interesting reading. :-)

This one compares 5 different dutched cocoa types and ghirardelli natural cocoa and how each one affects a specific brownie recipe.