Better options for pricing

[Nixill]

There's currently a single option for item pricing, which is just "how much should an item the mod takes away be sold for"?

This should probably be split into four options:

• "Should items be purchased for the mod? If so, for how much of the price?"
• "How much should an item that the mod gave be sold for when it's taken?"
• "How much of normal should shop prices be with the mod?"
• "How much should an item that the mod didn't give be sold for when it's taken?"