The reason you can’t find an answer is there are a lot of different answers depending on the flight, for the A 330 900, there were a few delivered without wifi because of supply chain shortages, i think those have mostly gone in for installation now, usually messaging is free, full wifi costs, that number is dynamic but my guess would be 20 to 30 dollars for the flight if you don’t get it free with t mobile, the crew is very good at announcing how to use the wifi for any particular flight, it is pretty straightforward