It depends on how often a router may disconnect; they typically receive a new IP address every time they connect. It's also a matter of the ISP's policy, some ISP's force a disconnection once per day while others don't.

> It costs 0.8 cents to update a namecoin domain That's not fair to say.

It costs like this because the namecoin price is so low. However, if it was used more, the price would skyrocket.

One of the main reasons to use a dynamic DNS service is because your IP address changes frequently. A service that requires you to pay for each record update is a non-starter for that use case.