Its kind of like the xp stopper guy, you pay to turn your xp off and you pay him again to turn it on. Its not quite as expensive (10g I think each time), not sure if it was ever higher as I never had use for him prior to the last alt I leveled. 10g isn't a lot (well 20 for the round trip), but if you do it every 10 levels or so, or every 5 depending on why you've decided to stay capped at whatever level, it will add up. Multiply that if you do the process on more than one character.

It seems void storage is going about it the same way, only definitely more pricey if these prices stay when it goes live. For someone who has lots of gold they might think ok, lets fill this thing up. Depending on how often they go back to get stuff out/put stuff in (if its a case of say transmog weapons.. new weapon type means a different transmog item is needed, gotta go get it out of the void.. the cost could add up. Its not like someone would throw 80 things in at once (ok, maybe thats possible) and at some point just withdraw all 80 at once. It'll more likely an over time gold sink. Where yeah, you toss everything you want to in there and once and dump that amount of gold. But then I think withdraws will be like one or two things here and there, maybe swap out some items for other items. Things you need once a year (empty brew steins, holiday costumes) those things will probably sit in voids and collect dust till the next holiday rolls around.. but armor/weapons, thats where the gold will get spent.