I disagree with this so much. I'm not saying there isn't some onboarding cost, but with experienced engineers its a lot less than you think. At worst you should be "breaking even" with a Senior Engineer almost immediately. When I look back at what I've done for the company I just started with 7 months ago, I completed a highly valuable, and large project for them in the first 3 months. I could probably get the same project done a lot faster now than I did then, but that doesn't mean it wasn't worth it to the company.