In my opinion, this is not a useful thing. I use lightweight backpacks for money.
Although it may be and only for knights. For me, 900 TC is too much. I play Druid and paladin and this is not the result for me.
I think few people bought it. I think the bag in which you can carry an infinite amount of money is not worth it. If it had an impact on weight, I think it would be a better option.
But for a knight who hunts in the library, this purchase is worth it. In the future, the purchase will definitely pay off. For smaller lvl I think it is not profitable.