I didn't even consider it being an icewine in the original post. That makes more sense to me with the 3 gallon amount for that price.
I'd also factor in that the juice from the kits aren't just juice you'd find at a store, but often a specific set of grapes from different vineyards. Varieties you won't really see at a store, but maybe either at a vinyard or sometimes at a farmer's market. I'm sure there's a bit of a price due to the processing, but especially in the beginning I'd prefer a decent kit compared to getting the grapes, crushing and pressing. Especially since if I were to buy grapes on my own, it'd cost more than the kit. I'd have to go in with a group who buys grapes by the ton to get a price. Maybe I'll convince a friend to grow lots of grapes or something.
The farmer's market here often has varietal grapes from different farms in the fall. I'm not entirely sure why they do since I don't know if they'd work as table grapes since they're wine grapes, but they're around 5 dollars a pound and if I can assume I need at least 5 pounds of grapes per gallon, I'd be looking at $150 for enough grapes for 6 gallons plus the work involved (hence buying in bulk).
It really seems to be the same with other fruits as well, trying to find the different variety to acheive what you're aiming for, as well as correcting the acidity, ph, sweetness and so on.
Now if I found places selling pasturized (not preserved) 100% reisling grape juice, then yeah, I'd go for it. But then, you'd probably be looking at something priced like the kit anyways.