The most xp efficient way to level heroes (prior to LB) is first to level to:
Legendary: 4/1, 4/80, 4/80, …
(and, I presume,:
epic: 3/1, 3/70, 3/70, …
and,
rare: 3/1, 3/50, 3/50).
To get a 5* OG from 4/1 to 4/80 requires 173484. xp.
To get a 5* costume from 1/1 to 4/80 requires 141426. xp.
It will always be possible to go back to level an OG at a later date but, as costume versions are typically superior to the OG, the main reason to do this may typically be for the final costume bonus.
sorry if this has been covered elsewhere, but this is the first time I’ve figured it out.