All Primary Monkeys in radius get tier 1 upgrades for free.
~ BTD6 description
Primary Mentoring is the fourth upgrade of Path 1 for the Monkey Village in BTD6. It allows all Primary towers within its radius to be granted free tier 1 upgrades. As of Version 17.0, it also grants Primary Monkeys with +5 range and reduces their ability cooldowns by 10%. It costs $2125 on Easy, $2,500 on Medium, $2700 on Hard, and $3000 on Impoppable.
For the free Tier-1 upgrades, Primary Mentoring is best used with Bomb Shooters, as their Tier-1 upgrades collectively are the most expensive of their kind.
The two most expensive Tier-1 Bomb Shooter upgrades are Bigger Bombs ($400 on Medium) and Faster Reload ($250 on Medium). However, the key value of the discount portion of the Primary Mentoring upgrade is when spamming in 2-0-4 Recursive Clusters, thereby saving up to $600 on Medium ($400 + $200, Medium prices) per Bomb Shooter.