$18,000 (or $36,000 as another comment suggests) seems incredibly low for "The largest ransom-ware infection in history"... how can this be worth it for the hackers?
Depends on where they live. For most people on Earth this is a very large sum of money that will go a long way (just not 'here' in the countries where the virus spread).
Since this happened over the weekend I am sure there are many business in for a nasty suprise on monday, as well users getting in contact with their "Technical Help" monday
So I bet their plan was to infect as many as possible over the weekend, and see money roll in come Monday once the Business Decision makers are in play and IT has had time to say "Yea our backups are fucked so we either pay or lose all data"
It started at 0700 on Friday, UK time. The infections to businesses in the Americas and EU has been done. Asia might be unscathed since the "killswitch" URL was registered. Until it mutates.
Lots of money might pour in on Monday as those infected decide that paying the ransom maximizes business continuity.