You have to contact ServiceNow. The community simply can't provide you any authoritative info on pricing, and pricing is determined by both the type and quantity of licenses that you buy.
An *instance* generally costs $15,000 / year, but that's not the minimum purchase. The instance pricing is almost always given in the context of non-production instances. You can't get a production instance without paying for a minimum number of licenses as well.
If you want to have an instance so you can develop apps for hte marketplace, then you'll want to talk to someone in the Partnership programs, not Sales.
