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

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.