Whoa, I'll stop you there. This is not a "data expense" problem. Nor is it a storage problem. It's a synchronization problem. I'll tell you straight up from a game I had a NDA with that there is nothing necessarily stopping the developers from having infinite space, but it comes at one extreme penalty.
More items you can have = larger packet sizes, too large of packet means fragmentation. If you know how and why MMO players tamper with the MTU settings, you'd know that the maximum packet size is just under 1500bytes. Those 1500 bytes have to be sent every 200ms (for 5fps of game play.) Within those 1500 bytes you have to store your character's coordinates, gear, physics, etc
So if you sit there and watch a packet monitor you'll note that opening, the inventory, picking up or dropping an item, or consuming an item, re-polls every item. Hence part of the reason FFXIV can do all these AOE dances in the first place is that the game doesn't constantly have to send updates to 100 inventory spaces every 200ms when the inventory is open.
So if you had say, 1000 spaces instead, now instead of 100 items fitting in one packet, it now takes 10, which means it takes 2 seconds to open or close the inventory, and everything else in the game grinds to a halt until it does. That makes playing dungeons impossible without "locking down" the inventory in some way so only part of it is available.
So here's where I'm going to suggest another idea.
Reverse how the Armory system works. Instead of having 25 items per slot, have one armory page for each class/job. While in a Duty no gear can be swapped except what is in the Armory for the class/job the Duty is for. The reason it works the way it does right now is to allow re-using gear between classes, which is all and good... if all your classes are the same level and you really have no desire to glamor anything. In V1.0 they removed a few item slots to reduce the complexity of the models, not necessarily the data sync requirements.
Then with the game's main inventory, have a "Field" and a "Dungeon/Duty" inventory. Field inventory consists of the current inventory we have. The Dungeon/Duty can only contain finished consumables (and item drops from the Duty.) When the Duty ends, the player can keep the consumables in the Duty inventory for next time, but the gear drops must be transferred to the field inventory, a retainer, or destroyed before another Duty can commence. The "field" inventory would not be available during Duty's.