My understanding is that the major part of the cost of a given model is the training - so open models depend on the training that was done for frontier models? I'm finding hard to imagine (e.g.) RLHF being fundable through a free software type arrangement.

No, the training between proprietary and open models is completely different. The speculation that open models might be "distilled" from proprietary ones is just that, speculation, and a large portion of it is outright nonsense. It's physically possible to train on chat logs from another model but that's not "distilling" anything, and it's not even eliciting any real fraction of the other model's overall knowledge.