overpay * 0.9 :)

so: price = 23 keys * 0.9 = 20.7 keys :)

If an unusual sale doesn't happen in pure, you must calculate overpay, and this is how it happens: 23k * 0.9 = 20.7, then round it to the nearest flat number, which is in this case 21 keys. Now, resuggest using 21 keys, and make sure that you write in the calculation.

When people sell items they almost always want pure but are willing to take "item overpay". To account for this you apply overpay by multiplying any non-pure item values by 0.9 to get the pure value of the item. However, given my experience with australium sales 20 keys is a very reasonable pure price for an australium black box with a spec KS applied so it really shouldn't matter in this case. The overpay, if any, on the Bill's Hat is negligible.