While it's true that 5G and OLED are going to cost something (there is also the rumored increase in base storage), you're forgetting that iPhone 12 is going to be a much smaller device than iPhone 11. It's going to be a successor to the standard 8. Smaller phones are priced lower. So, 12 Max should be priced like 11 with all else equal and 12 should be cheaper. Sure, they can do this. But it's hardly a question of cost, especially if they were to cut charger and headphones. The only reason I can think of is making space for a 4G version so they can maintain separation from SE. And, of course, padding their margins.
It's a similar story with the cell. 11 should be compared to 12 Max. Not the base 12. A much smaller phone. Smaller phones are smaller, imagine that, offering less space for a cell. They also consume less power, especially if they also have lower resolution. So, they don't need as big a cell.