I'm not quite sure if the OP meant justifying open sourcing their tools or writing them in the first place. However, for open sourcing, I'm pretty sure I could

I'm not quite sure if the OP meant justifying open sourcing their tools or writing them in the first place. However, for open sourcing, I'm pretty sure I could justify that pretty easily just on reducing the cost of recruitment. There was a time that Facebook had (at least in my circles) a terrible reputation as an employer. Working at Facebook would be a laughable job. Now it's one of the most sought after jobs in the industry. Not only that, but people regularly spend time getting familiar with their in house tools before they even send in a resume. For me, it's basically a no-brainer.