An efficient method is presented for signaling link price information using single-bit marks. It exploits side information in the IPid field of the IP header to allow the maximum price on a flow's path to be estimated. It automatically adapts the resolution with which the price is quantized, depending on how quickly the price changes. The algorithm does not depend on the number of hops in a link. A marking scheme with improved compatibility with current ECN (RFC 3168) is also proposed.