I think 15$ is over the top, let's not forget this is a private shard. Even some AAA MMOs charge less for a subscription or "premium" account. It'd say something along the lines of 5-7$ monthly would be a good price to pay.
Citadel doesn't have the files to open a server like this, all their other off files are in very crude state and would takes months upon months of parallel BETAs in order for them to even consider opening one.