The InTheHand.Net.NetworkInformation namespace has functionality to access network statistics on a per-adapter basis. e.g.

using InTheHand.Net.NetworkInformation;
NetworkInterface[ interfaces = NetworkInterface.GetAllInterfaces();
//check the name properties to determine which interface to use
IPv4InterfaceStatistics stats = chosenInterface.GetIPv4Statistics()
int sent = stats.BytesSent;
int received = stats.BytesReceived;
//etc