On SubetaLodge it says that the estimated user price is 9,000,000 sP

Not sure if that helps but thats all I could find!!

That background comes from the Legendary Chest (female). It's not in the cash shop now, but when it was it sold for 500 CSC. Even though it's been retired for a while, it's possible that it can come back at any time (after all, this is Subeta) and it's probably not advisable to pay more that the cost of the trunk.