Depends. Erlang's scheduler is clever: if the receiver is asleep waiting on its inbox, sending to it will switch the running process to the receiver and back, all without hitting the scheduler logic. This is a great benefit for "thread" (in Erlang it's a process) pools, which are very very widely used in Erlang programs. It's really the best of both worlds: no Erlang context switch and it's completely transparent to one's codebase.

This isn't possible if the receiving process's inbox is non-empty or it's running. Then the cost should be around the same, assuming your C++ implementation is at the same level of quality as the Erlang VM.