There are no dollars in the laws of physics. It's connected ultimately to productivity of all the activities involved, and there's no obvious upper bound to productivity.

Just the current market price for fossil methane, which of course goes up and down. But I'm assuming SpaceX gets their methane on the open market.

It's quite possible that SpaceX has access to some cheaper methane source. Texas produces a lot of it.

And SpaceX has speculated about eventually switching to some renewable source by e.g. synthesizing methane. In which case that would boil down to cost of electricity and carbon. I don't see that becoming cheaper short term but that could happen long term.