A domain name for your WordPress blog usually costs in the neighborhood of $9-$10 per year, depending upon which company you register it with, and whether you use a coupon or not.
Keep in mind that there is no distinction between a WordPress domain, so to speak, or a domain that uses any other kind of software to power its content. Whether you use WordPress or Joomla! makes no difference with regards to the cost of the domain registration.
If you are currently hosting your blog on WordPress.org or another free blog site such as this, I suspect that you will find the $10 per year a relatively small price to pay for the freedom that having your own domain will allow.
You will get full control of your WordPress installation, and be able to monetize your site in any way that you see fit without any restrictions. You can also use whatever custom themes and plugins you prefer.
When buying a new domain name or renewing an old one, I highly recommend that you search for coupons and discounts on Google before you complete your purchase. Using coupons will usually save you quite a bit of money on the cost of domain registrations, and it only takes a minute or two to check for them.