I read through this thread and had a few comments.
On the issue of some items costing too little or too much, that is why there is a DM for the game. A +2 to +4 too much utility (e.g. Constant Improved Invisibility item) or -2 to -8 not enough utility (e.g. permanent rose) Spell Slot modifier (or other advantage or restriction) can correct that problem right away. Everyone knows that there are some spells that are too powerful (or have too much utility) for their level and others that are too weak. A slight adjustment by the DM on those spells handles the problem nicely.
On the issue of varying costs with the DMG, that's really ok if you think about it.
If you consider a CP to be a dollar, a SP to be $10, and a GP to be $100, a 50,000 GP creation cost is $5 million to create, $10 million to sell. Should 12th level characters
really have $5 million in cash to lay down for an expensive item? Will they find someone willing to part with that much money to buy said item? Well, it depends on your DM, what and how many items (and other treasure) he gave in the past, what you can sell them for, and other game economics. But, if your DM is on top of his economics (i.e. does not play a Monty Haul game), there shouldn't be a problem if one item costs 10,000 GP in the DMG and only 6000 GP using Spell Slot (or vice versa 14,000 GP using SS). As long as the DM is consistent and fair, the costs should consistently be equitable with regard to other magic items in the campaign world. The reason is that each item is priced based on caster level, spell level, and utility for each effect in the item.
So, although Spell Slots themselves limit what items can be created at a given level, so does the amount of cash the PCs can get their hands on, again based on the prices they can sell other items for (and how many opportunities the DM gives them to do that).
Ok, just my two coppers worth.
