Let's take a look on router Linksys WRT54GL. Powering on only power supply (old type, transformer one) uses 2.5W of power. No router connected yet.
Then I connected two computers over Wi-Fi connection and plugged ethernet cable from my ISP's modem. Energy use increased to 6.9W when computers (network) were idle, and increased further to 7.5W during sending data over wireless connection.
Didn't perform check with ethernet cables other than WAN plugged in.