Depending on the agent, you might get lucky in YVR that they will let you take 1 bag if it is lighter than the maximum allowance (70lbs - health regulations) for a single bag. If not, you will have to be around 25US$/Euros.

In Amsterdam the chances of getting an agent to allow you do that are sadly non existent - I know from experience! Of course it's my own fault for packing that ways well and for expecting some generosity from KLM!

In short - if you know you will be coming back with the same amount of stuff, split it across two smaller bags, or be prepared to pay extra twice.