I didn't try it, but I guess there would have to be some more brackets around each step of the way so solve this with a long long long one-liner, but reading that hurts anyway. Would it be horrible to use the long version?
if event.get('value').nil? then
event.set('field', nil)
elsif event.get('value') < 1000000 then
event.set('field', 'Low')
elsif … etc etc etc …
…
end