"There's also the overhead of maintaining the ability to manufacture new ones, regardless of how many they can reuse. Retaining that manufacturing capability whilst it's sat idle is almost as expensive as just building new ones all the time. "
You have to balance production capacity with launch capacity and the relevant economics. And then there's the refurbishment, which will probably done in the same production facility. If you can get enough orders for launches at 2/3rds market rates then you need to be producing replacements at about 1/3rd the launch rate (assuming you only use each 1st stage 3 times) These are numbers I'm sure SpaceX have crunched in various ways.; I think Musk has stated the target it is least 2, maybe 3 launches by pretty much just refuelling a landed 1st stage. After that, not sure if it's cheaper to scrap and replace or to refurbish.