Sometimes you don't have to pay anything at all, but that's usually when you have well-known players in the server already, then people just join on their own, without the need for a global.
For decapprize, it's not necessary either, but it does work as a good incentive for people to join.
Overall, if you want to spend tc so your server has better chances of picking up, 10k for global (5k if you find a vip), and 5k to get the dp started.