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.

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.

> 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.