I want a mount gift as a reward for 10,000 QPs.
Why 10,000 (math work)
Going by the 5 costumes for the price of 4 deal, each costume is worth 268 Minecoins. Since costumes can be purchased for 1,500 QPs each, we can say 1 QP is worth ~0.179 Minecoins or 1 Minecoin is worth ~5.597 QPs.
Just take the price of a mount(, 1690 Minecoins,) and multiply it by ~5.597. You get ~9,459 QPs. 10,000 is just a nicer number though.
I guess there could be similar methods of pricing for other hub cosmetics, but I just want a mount.
I have priced the other hub cosmetics here: Quest Points Redeeming method - #3 by RicOnSwitch