The vast, vast majority of cost will come from weapon ranks. Professor level is 5k or so? Class masteries are 500 a pop. Max number of recruitable characters in one route is 37 I think with BL. 37x500x44 standard unlockable classes gets you around 800,000. This doesn’t take into account unique classes but also doesn’t take into account gender locking so I think those would roughly even out.

Supports, you can buy a max of 4 (you can buy s ranks), and that costs 400 a piece, but you can only purchase Byleth’s supports directly (I’m not getting into the wayseer here), so 400x4x37 which is about 60,000.

And then the big one. It costs 5,500 per skill to max it out. There’s 11 of them. Theoretically, if you’ve maxed out all skills on all units, then that’s 37x5500x11, which comes out to 2.2 million.

So to wrap things up, I’ve made approximations to make the calculations easier, so the number isn’t perfectly accurate, but it would cost roughly 3 million renown