Consider it as follows: To create an item without spending XP, you have to spend half its price in crafting cost (it makes no sense to waste dust on that) plus 20% its price for the XP (this is assuming a party member provides the spells). At 5% dusting efficiency, that means you have to dust items worth 4 times as much as the item you want to create. You could otherwise sell those items for twice the price of the item you're creating, so making an item via using dust for XP and nothing else costs 2.5 times the price of buying it. Most of the time, it'll be easier to find someone to make it on commission for a 50% or even 100% markup.

If you want "dusting" to be at all feasible (and also make the dust prices realistic), you want:
-the dusting to give between 50% and 100% the item price in dust value (less and selling it to buy dust is always superior, more and dusting your items to sell the dust is superior to selling the items; in between and it's only worth dusting if you want the dust)
-The dusting to be worth XP at a cost of less than 12.5gp per XP (at 12.5, nobody's going to want to buy dust, as the dust cost to avoid spending any XP will make up the difference between the crafting cost and sale price.) 5GP per XP seems pretty good here.
-Replacing spells should be far less than 500XcasterlevelXspelllevel, as you can hire someone to cast it for you at a cost of 10XcasterlevelXspelllevel per casting. So it should be at less than 10XcasterlevelXspelllevel for each time the spell is cast (in most cases, I think you have to cast each needed spell once per day.)