Earlier quoted context omitted.
For what it's worth, someone on reddit says $12 https://www.reddit.com/r/news/comments/9kpeal/amazon_raises_...
Quick back-of-the-envelope math: Let's assume this raise will affect 250k workers this coming holiday season, bumping their pay from $12 to $15 (+$3). Over 12 weeks at 40h/week, this represents an additional $360M in labor cost. Using AMZN's 2017 revenue of 177.9B, this equates to 17.7 revenue-hours.
Amazon's 2017 financials showed a final net income of $3 billion. They do reinvest aggressively in their company so a fair chunk of their costs are going to be 'voluntary', but that voluntary expenditure is the very action creating all these jobs. In any case, this is going to be a very substantial cost to them.