So, the challenge with a retrofit is this ...
#1 - the windshield glass usually requires a special reflectivity coating. For example, on my vehicle, there are two windshield part numbers - with-HUD and without-HUD. I had to replace a windshield and needed to be *very* specific to use the OEM, HUD windshield, otherwise, the projected display is dim and fuzzy.
#2 - Given the dash design of the Model 3, where do you put the projector, without interfering with the HVAC vent? This would also require a separate data channel connection, which means a new wiring harness, etc.
It's not quite as easy a retrofit as it would appear, I'd say. Sure, at volume, it'd certainly be less than $200. But it'd be a LOT easier to retrofit into, say, a Model S/X refresh, or the brand new Model Y, than as an inline engineering change on the Model 3. An after-purchase upgrade would be even more difficult.
Moreso, as nice as it is in my Cadillac, I don't think it gets the hype one would expect. It's always a "hey, that's cool" feature but not something that I think sells on its own. It's always part of a bundle (akin to the Premium interior.)
So - would adding a HUD increase Model 3 sales? That's the golden question.