If you add a subdomain for $5, it has all the features of your hosting plan.
If you get another plan for $10, it doesn't have CGI or MySQL or secure hosting or any of the other features you may need. You spend your life logging off one account and onto another, instead of being able to manage all the websites through one account.
With multiple domains on one account, your CGIs can share data easily using ZERO bandwidth, and with some forethought, they can share code as well.
And if you are paying the $5 for another domain, it doesn't have to be a subdomain. It can be a completely different domain. That means you can host www.customera.com and www.customerb.com on your plan at a cost to you of only $5 each, which leaves lots of room for markup without pricing yourself out of the market.