my view is this:
~4.5 (easy to use for defense, but slow and not great for offense)
~9.5 (good for defending and attacking, forking, and can threaten many other higher value champions)
~14 (begins to behave similar to queen near center of the board)
~17 (acts as queen in center 4 of the board, with tiny overflow)
~19 (acts as queen in center 16 of the board, but more overflows)
~20 (more overflow)
~21 (more overflow)
This is why adding +1 attack range as a final upgrade usually comes with some extra swap utility or move bonus, I feel like the longer range something is the more likely it is to overflow off the board entirely and not be used, other than instances of the unit being used for rushes.

Honestly the last 3 are a weird crunch, and going backwards I probably would have started with 21/19/17 from the visuals alone. This problem in my list is most likely started early on the first 3 being so steep, rather than being a problem with the final cost of 21, but I think the gradient decline in value per range is fairly accurate in most cases.

Also, melon should be cost 4 at least, because Greed.