I can agree that many on Amazon are a lot cheaper. But if you have never used the XP backpack how can you say "many are better"? I am sure your Huntvp suits you fine but it is too small for my liking and it lacks many of the features the XP packs offer. I've never had one of those Huntvp packs so I can't really compare its build quality to the XP pack. But if you re happy with your $24 pack that is all that really matters.
the $24 one is "convenient". Small, light, easy to handle , just like the Deus 2. It allows me to carry my pointer, another detector, and a few other things. there's a certain point where, if you have to load up a whole heavy backpack up to go metal detecting, it's turning into work. And.. I hardly know it's on my back, it's that small! But it does what I want.
As for build quality.... you are paying for a specialized product that cost a lot to make with the XP pack, vs these other backpacks sold in the 10's of thousands, which brings there per unit cost down. You are not necessarily paying for quality, you are paying for a low production specialized product.