There are two costs related to domains: domain registration, and service. Domain registration is an annual fee, that gives you the right to use the domain until it expires. If you register your domain with Pobox, the cost is $20/year.
You can also register it elsewhere, and use it with Pobox for service.
There is no additional cost to add an address at your personal domain to any existing account. However, the costs related to service (either the cost of the account, or the cost of additional aliases/addresses, if you wish to use them) is the same as if you were using a Pobox domain. You may have up to 100 on your account (up to a maximum of 100 total addresses per account -- you can't do 100 on your first domain, and another 100 on a second domain.)
The only other domain-related charge is AllMail, which is $10/year. AllMail collects all the mail sent to any address at your domain. As you might guess, that means processing a lot of spam. Hence, the fee.