Book Cover Commissions?

I have been thinking of offering them to supplement my income. (I want to start playing with digital art, and I have my eye on a graphics tablet that requires a bit of extra spondoolix.)
Of this sort of variety, for use on DA, Wattpad, FictionPress, etc. Extra charge for commercial use (elements having to be purchased and all that).




Cost would include finding, modifying, and occasionally creating graphic elements, checking licencing permissions, and spiffing things up in general.

How much would folks be willing to pay for something like these?

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