Honestly I think $3 is fine for additional characters...if you need more than 1 additional character as a 'mule' type, well you might need to consider being in an episode of FFXIV hoarders ...

By making each additional character cost $3 dollars on your account SE is pretty much saying 'You can have a full army of mules, but it'll cost you.' At the same time, SE knows most people won't mind paying an extra 3-6 dollars a month for 1-2 'mules' that might operate as LS banks and/or storage mules. I get how this can be upsetting, but controlling how much inventory each player has is a way of controlling the in-game economy. Right now, people can just stockpile every mat, Rare/ex drop, and 'maybe I can use this somehow' item they desire. That just means that there are less mats and lower end crafted mats/items for sale on retainers (if you lack storage you sell/NPC/trade/ drop more stuff). But by charging $3 for each additional character, it will force some people to decide which is more important...more cash in real life, or a better stockpile/more gil in game.