An item that would work like Magical Mallet (for the first hand only). Something that would allow you to swap out either a predetermined amount of cards (maybe somewhere between 2-3, with 2 being a mandatory minimum), or, to just mulligan and try for a new hand.
I know I've got problems with drawing/top decking on Dueling Network >_>
A cost would be dependent on the way it'd be used.
If it were for swapping out 2 or 3 cards as listed above, probably somewhere around 150 or 200 DP to purchase. Being able to choose your cards (sorta) would give you a bit of a leg up.
If it were to get a fresh hand, maybe anywhere from 100-300 depending on how the admin's feel about it's usefulness.
Then again, I was never good at guessing prices. You guys can go nuts with the cost however you deem fit.