Technically they are only slightly less affordable than regular domains but that depends on whether or not you are identified as a "Local" or "Foreign" entity. Unlike most other domain names, pricing for .NA domain names vary depending on whether or not you are Namibian.
As a local, you would currently pay R 750.00 per year for the registration of a .COM.NA domain name, which is about 45 USD. However, if you are a foreign entity, this price shoots up to R6500.00 per year (almost 10 times as much). The regulator of .NA domain names decided on this pricing and have kept it working this way for several decades. It is therefore unlikely to change.
This pricing structure is especially important to keep in mind if you are purchasing a domain name from someone else. If they are Namibian, they might enjoy the more affordable pricing but if you purchase it as a foreign entity, you will be obligated to pay the higher fee.