I think that you are likely correct but with a possible nuance I have a theory about.
I have a new team that has zero homegrown players, but is all of the home nationality of Scotland. That would mean the above should have my team sales and the homegrown & local merch being the same. Instead homegrown & local is 86% of team sales.
My theory is that the equation for a player starts at 0.8x for Local and 1.2x for Homegrown (or something like that) in their 1st week and then the longer a player stays on your team, the nearer it gets for each player to the 1x and 1.5x you calculate it with.
This would explain why your estimate would be slightly low, plus it makes sense that fans buy a bit more merch for players they know have stuck around and therefore will stick around. I'd be annoyed to buy merch for a player who left the next week and maybe fans in BuzzerBeater have been programmed likewise!
Therefore if my team sales and homegrown/local merch totals start getting proportionally closer in weeks where I haven't bought a player then I will know that I might be on the right track!