If they have $600MM revenue, then even applying a modest 5x multiplier would give them a $3B valuation right? And 10x might be closer to what they might actually fetch if they were to ever sell.
And if Walmart had the assumed 30% margins, they probably could be a $2.5 trillion company. Their revenue is more than 2.5 times their almost-$1 trillion competitor.
Facebook bought instragam for $1 billion even though there was no significant revenue or income. But the potential for growth of instagram itself and the photo sharing social media space was immense.
With established companies like walmart in establish spaces like retail, you know what you are getting. With tech unicorns, it's a bit of a gamble.
Look at marketo for a good comp: 21x EV/TTV - http://tomtunguz.com/marketo-vista-ma/