Never tried to measure it, but it'* not much with a 180. Drive like a civilized person on the interstate and I doubt you would see any degredation in mpg. The colder you go, the greater the chance your car will get stuck in open loop, where it never tries to tune the motor to a leaner mixture. That'* why some of the chip makers call out using a stat that opens at a colder temp (They have already removed/lowered the threshold temp for the motor before it will go into closed loop.) But a 180 isn't that radical a change from the factory base operating temp. A 165 would prolly keep the engine in open loop indefinately, and the gas milage would really suck (say like 18mpg highway, belching smog all the way.)

Low ambient temps shouldn't present a real problem. Your car runs ok in your climate now right? So, the motor is capable of getting up to something approximating the operating temp range now. If fact, cold, dry days are the best thing for a motor to generate it'* peak power output. (Cold and Dry = very dense air being sucked into the manifold, which means the engine is sucking a perfect air charge into the cylinders) The MAF and IAT sensors will tell the ECM just how good the air density is and it will adjust the mixture accordingly.

If it is so cold outside that the engine can't even make it up to 180 with no coolent flowing, then it'* probably a good day to stay home with a bottle of brandy, less you accidently drive into a cravas in the glacier