The thing about the cost of rocket launches is that it's not necessarily all that important, depending on the context. For launching a bunch of relatively small satellites into low Earth orbit, launch costs are one of the biggest costs, especially when you're looking at the trend for small groups with similarly small budgets wanting to send up things that you could fit on your desk. And for that sort of thing, SpaceX and others pushing for significantly reduced launch costs has been a big deal.
But for larger missions to send significant payloads to the Moon, Mars, and other distant places, the cost of the actual launch is a much, much smaller proportion of the overall cost. The Mars Science Laboratory mission (including the still running Curiosity rover) has so far cost in the region of $3 billion, with the launch cost being around $140 million. If they'd used a Falcon 9 instead, it might have saved maybe $40m at best. This is why NASA has tended to focus a lot less on worrying about the cost of rockets. No-one looking at manned Moon or Mars missions cares if Falcon Heavy or SLS turn out to be cheaper, because as long as they can get the job done the difference in launch cost will be a rounding error on the total cost of the mission.