Academy Book 6.
Level 1: 40k wood, 40k iron
Level 2: 80k wood, 80k iron
Level 3: 200K wood, 200k iron
Level 4: 400K wood, 400k iron
Level 5: 800k wood, 800k iron
Level 6: 800k wood, 800k iron
Total = 2320K wood, 2320K iron
Average drop is around 2.5K wood and 2.5K iron. In other words, you need to plunder 928 times to maximize the skill.
To max train a hero you need on to train him/her on average 220 times. Each train requires 6K wood and 6K iron (mercury should not be an issue).
This means each hero requires 1320K wood and 1320k iron. In other words, you need to plunder 528 times to max train a single hero.
If that is not grinding then i don't know what is.