dry.rb

day = case self.type
  when 'fast' then :fast_completion_day
  when 'super_fast' then :super_fast_completion_day 
  when 'ludicrous' then :ludicrous_completion_day 
  else :budget_completion_day 
end

self.value = Increment.first.max_hours * Incrementor.first.send(day)

prefix = %w[ fast super_fast ludicrous ].include?(self.type) ? self.type : "budget"
self.value = Increment.first.max_hours * Incrementor.first.send(:"#{prefix}_completion_day")

This is probably about as short and dry as you can get it. That said, I'm not sure that this is more clear/easy to read. I'd probably add a method to Incrementor like Incrementor#completion_day_for(type) that would take care of this logic for you.