Tier 10 Foeripper goes for 30k, and that's a class above this.
For a tier 10 Slugstorm I would think maybe 20k, the higher end premium price would be 25k, that range seem reasonable to me.
Price is going to depend on the open tiers though. My estimates are based on tier 10's, as I assume that's why you'd want this gun... fun little event gun.