Since you can cooperate on magic items, dust cost should be more than 10gp * spell level * caster level (the amount to hire another caster), but less than that cost + the cost of finding the caster in the first place (in time and/or money). It would also be prudent to make it cost more than a scroll, which is 12.5gp * spell level * caster level.

I feel like dust giving 25% of market price is good, because that means you can dust an item to get the maximum dust you could use on an equivalent-value item (except to replace spellcasting). However, you could just sell the item for 50%, then get 25% in dust and the other 25% in cash you need (same problem as Yitzi brought up). It feels forced to say that you can't find dust anywhere on the market - if this is so useful for enchanting items, why wouldn't it be for sale? The quick answer is that dust is specific to the disenchanter, but that's just a crafting reserve with a different name.

Also, 1lb per 100gp of dust is a lot of weight. Using your example, you put 67.5 pounds of dust into a 6lb axe, and can remove 20lb of dust later (at a 25% rate). Magic shouldn't break the law of conservation of mass that badly. 1lb per 1000gp is probably okay, but still leads to +5 weapons returning more than their weight in dust.

[hr]Deer version:

15-25 gp * CL * spell level is an acceptable price to replace spellcasting.

Dust returns should probably be at 50% if dust is available on the market, and there's no reason for dust not to be on the market.

1lb/100gp is too much weight.