I live in rural KS and have two dawn-to-dusk 175 watt mercury vapor lights on my property. I've been looking for a more energy efficient option. I like the amount of light and the durable housings of the mercury vapor lights but don't care for the high electricity cost. Any suggestions? I don't want to spend an arm and a leg to replace. Thanks for your thoughts. 

There are both flourescents cabable of 120 watts outputs
http://www.buylighting.com/Compact-Fluorescent-Flood-Lights-s/114.htm


and LEDs available

Fluorescent lights used in cold temperatures may require a special ballast.  Consider LED's instead.  Longer life - more efficient.

As proven this winter, we get quite a lot of cold weather in the winter. I'm not sure the CFL's would do well when temperatures go below 20 F. I'm intrigued by LED and am unaware that LED's have been developed to throw the same amount and pattern of light as a traditional mercury vapor light. Any more info on where to find this option would be appreciated.

I would look at a smaller high pressure sodium, maybe a 70W would do the trick. Right now high pressure sodium is still the most lumens per watt (light out per watt) in an off the shelf item. They are about twice as efficient as mercury vapor, so halving the wattage you should still get the same light out. Compact fluorescents typically fall between mercury vapor and sodium lights. Led’s can be more efficient, but to get that many lumens out your looking a custom fixture $$$.

I have seen low temp cfls for sale. I can't find the lik right now but two years ago they were $6-8 each and would start in temps as low as -20. I'll continue to look for the link.

Not the site I was looking for but http://www.sylvania.com/AboutUs/Pressxpress/Pressnews/DuraOneExpands.htm supposed to start at temps down to -20. Another option is LED, very expensive up front costs though. Go to cree.com and you will find some partners that specialize in public area lighting with leds.

Mercury vapor typically runs with ~45 lumens/watt (+/-) efficiency.

High pressure sodium runs ~110lm/w, so you can cut your power in half and get you get the same luminance, with similar color rendering (albeit a different color)

Low pressure sodium is higher efficiency still but has the worlds crummiest color rendering- it's more light, but due to low CRI the experienced visibility to the eye is somewhat worse. Black & white security cameras can use it though.

Metal halides run about 80lm/w, and have considerably better color rendering- you can probably run about a quarter to half the power of your mercury lamps and get better overall visibility, if naked-eye (as opposed to camera) view is what you're after.