Cheapest?...i hardly think so,while High Pressure Sodium is More Efficent
This is the main reason for the "cost of light" difference
the Fixtures,Lamps and gear cost more than an similar mercury vapor installation
The real difference is only the ignitor, other differences are only marketing. So when MV's would enjoy the same popularity (between the lighting officials) as HPS, the gear would cost the same.
mercury vapor lamps dont Cycle,or Leak...id rather have a dimmed out merc than a HPS that was Cycling,The Falliure rate of HPS is Higher than MV,not to mention theres less usable visibility from the Ugly Orange Light it emits!
Well, if the HPS would be serviced as HID's should (so maximum 4 years group relamping for newer HPS, otherwise 3 years for both HPS and MV and 2 years for MH), you would nearly not see any cycler, nor dimmed out merc. By the way merc with output below 70% of rated level (so barely not yet noticable brightness loss) belong to "not working", exactly the same as HPS cycler.
And the ugliness of the orange light is matter of quality and not cost of the light, then it is other story.