Both registration and renewal fees help fund the continued development of the ENS protocol. The cost for renewal of your .eth name is currently the same fee structure as registering a new .eth name.
In addition to the renewal fee, the Ethereum network will require a small gas fee to update the data on the blockchain.
The cost of an ENS name is dependent on how many characters it contains:
Number of characters
3 and 4 character names have a higher pricing to reflect the small number of these names available.
All .eth names have an expiration date, and in order to maintain ownership of the name, it must be renewed so that it has a valid expiration date. Additionally, once an ENS name expires, there is a 90 day grace period in which the name can still be renewed and the owner will not lose the name.
Any Ethereum wallet can pay the renewal fees to extend the expiration of a .eth 2LD (eg. name.eth); it does not have to be the owner of the name.
The Name Wrapper contract makes it possible for parent .eth owners to create their own subname registrar and have their own renewal fees they set themselves as the owners.
ENS does not dictate renewal fees on subnames, only renewal fees of .eth 2LD names (eg. name.eth).