It is expensive, but it should last for many years and once the initial sting of that cost surpasses, you have a bag that can stand up to the worst of conditions. It’s worth it in the long run.
I managed to pick up an Orvis Galeforce rucksack a couple of years back for less than half price as it was making way for the new model
https://www.orvis.co.uk/p/orvis-waterproof-backpack/2hnyI don’t tend to use it on the rivers as I like to travel lighter, but for lochs or on a boat it’s great. Fill it up and dump it nearby and you don’t have to worry when it starts chucking it down, knowing your stuff is dry.
Also that’s one expensive item I personally wouldn’t class as a tackle tart item.
Fully waterproof stuff is expensive for a reason. You can buy a £70 rucksack but your stuff will be soaked in a downpour.
Now I don’t tend to head out in overly wet weather, but sometimes you get caught out and it’s nice to have dry stuff,