[pdovy]

It feels like there is an attitude here that you should be able to recoup the cost of training a junior hire by paying them below market rate for for their new skills. This just isn't reality, as OP has discovered.

That means that the true cost of a junior candidate vs a senior candidate isn't just their 1st year salary difference on paper, you have to count in the cost of training _and_ retaining the junior hire. That doesn't necessarily mean the math doesn't work - especially if the market for senior developers is much tighter than for junior developers.

Also the idea that you'd be mentoring for 16 hours a week one-on-one seems like it is either inflated or there was a problem. Maybe the candidate was a bad hire even for a junior role, or the mentor is micromanaging, or the work product expected was never appropriate for a junior developer - but something seems off there.