The DNS lookup only has to be done once and is then cached on the users local machine, at least for the overall request (if not longer). Thus the cost is only

The DNS lookup only has to be done once and is then cached on the users local machine, at least for the overall request (if not longer). Thus the cost is only 160 bytes for the page, which isn't all that much. Additionally assuming their cache timeouts are non-trivial there are several caches to reduce the delay incurred by this.