I was advised by a senior manager in flight ops, he who knows the cost, that if you need to update enroute WX then don't hesitiate. The cost is miniscule compared to the total communication costs pre flight.
Many parameters are sent without our knowledge all the time! Some to EK and other messages to the manufacturers of the airframe and engines.
Therefore one or two added WX requests may only add a few extra kbs to already hundreds automatically sent without pilot knowledge.
