This paper proposes a cluster-based routing scheme for improving cost-effectiveness of message delivery in delay-tolerant networks. The proposed scheme is designed, as follows: (1) every node calculates the expected node density in network proximity; (2) every node perceives whether it stays in a cluster or not; (3) every node can ask for relay nodes to carry messages in a cluster; (4) a network system can control the quantity of message copies by measuring the message density of a cluster to help message distribution in a network. Performance under synthetical simulation shows that this scheme is able to obtain comparable effects in terms of delivery probability and message overhead ratio, particularly when the mobility models are in accord with human behaviour.