From page 10 of the 2015 Form W-2 instructions (pdf):
Of course, there may be state laws or regulations governing this.
Federal tax regulations require the employer to provide one free copy. That’s it. The rest is between you and your employer. Your employer is not required to provide employees a free document retrieval/copying service.
This is an increasingly common practice among employers. Generally they charge a small fee for current W-2s and progressively larger fees for older ones. $50 is an extreme fee for a current W-2, however.
If it were the fault of the employer that you did not get your W-2, you could make a case for making them correct their mistake without charge. But it looks like they are trying to cover their behinds by giving you the opportunity to correct their records.
If it’s an option, I would suggest signing up for electronic delivery. Not all employers offer this option.
ETA: I type slow. Looks like it’s already been answered. Twice, no less.