OBS Studio is free. any cost would be if you purchased certain add-ons/plug-ins, and certain services (like restreaming)
There are free plug-ins in the Resources section for streaming to multi-platform
However, beware of the implications... possibly double the real-time video rendering, and double the bandwidth.. because of that not inconsequential impact, there are services where you send a single stream to, and for a cost (Restream.io, AWS, others) they will re-encode/forward to your stream to multiple platforms at same time
