If your domain host charges for sub domains, find a new host, that kind of service should be free. If you are not sure where to find one, check out zonedit.com . Their domains update in almost real-time, and they give you plenty of control. They also have guides for how to add sub domains. When you make a subdomain it will point to an IP address, this can be any public IP address, including ones that you used on other sub domains.
As for the costs to a business to add a subdomain. The cost is so small it is legally immeasurable. It is adding a few lines of text to a config file in a server somewhere. Charging for this server is a rip off, instead of charging for domain hosting as a whole.