The cost of registering a domain of Namecoin include two parts:network cost and fee.
This will be charged by Namecoin.The first registration will cost 50 Namecoin and the network fee will decrease 50% every two months that means the network fee will gradually drop to less than 1 Namecoin in a year.
The reason of spending so much to register a domain at the beginning is to set a limit to make sure we have enough domains to register in the future. But as time goes on, the network fee will drop to be negligible.The network cost will never run into anyone’s pocket because it is destroyed with transactions.
Meanwhile the fee will be sent to miners which is similar to Bitcoin.You can decide to pay how much fee to miners which can be 0.01 Namecoin or even zero.But the more you pay,the faster your transaction will be done.
You have to upgrade your domain every 3 months and the process is free.