The spawner price should be set at 50k, not 15k. 15k is far too low, and if you want to compare prices in the admin shop, other spawners found in elite/supreme shop are equivalent in value. Thus such a pricing contradicts making it a special item. The rabbit spawner is as I mentioned is in the ultimate shop and cannot be /spawner'ed. It costs 50k per and there are no issues with rabbits' feet being too cheap or whatever. If made available as such then there will not be any real problems.
If it is at limited sale then there is no purpose in that either. Either we all fairly get to availability to purchase creeper spawners (as ultimates) or we get nothing at all. No need to get into bulk pricing either if you know how the admin shop works.
I.E. Don't complicate/over-think it.