No, the registration and renewal fees are the same price, and subdomains are free. Domains are dirt cheap everywhere unless you run into a scam registrar.
Now let's get back on topic before we derail this thread any further. I may make an fg.uboachan.net for fangame projects. However, I will never follow MsWoot's suggestion and split the site into multiple imageboards. That's a really terrible idea and it would kill Uboachan instantly, I guarantee it.