-i agree bag slots are expensive considering they're character based (which is stupid imo considering the price : 400gems, ie half the price of a bank tab/char slot... really?) which is why i wouldn't advise anyone in buying one unless you've got so much gold you don't know what to use it for.
-bank storage : i second that but well, it's a lot better now since they added more mats tabs (halloween update i believe?), but i too am struggling with that with all those rares i keep for my alts or for later use in forge.
-char slots : i bought 1 in the first month when gems were cheap, bought 2 more this weekend as they were a tad cheaper (600 instead of 800 gems, cost me like 17g i think which is pretty cheap - i only got 100g at the moment, not that i'm that rich either)
-mystic stones : i don't buy them, still got black lion salvage kits from quests rewards so i'm good for now.
-20 slot bags : you can get 3 for "free" (huhu) using relics from fractals dungeon. it takes 600 relics to buy all three. 600 relics is nothing past some point, like level 15+ i think. other than that, well yeah, 20 slot bags are not much worth it and are one of the biggest gold dumps of the game i guess. 15 slot bags though are fine for their price imo.