I wash my plow truck at a coin op. I usually pay about $8 Canadian for about 9 minutes with a sprayer, I wash off the truck, plow, salter, and then thoroughly go over the underbody. That's usually enough time to use a spray soap and a rinse, but really you only need to rinse.

If you're using it for plowing, that cost, every storm is nothing compared to the amount of money you'll save on the frame and body over time. Water isn't what causes the rust, the salt is what really accelerates it from the road. Whatever is used to melt ice. You need to get that off immediately, before it has time to set in. Especially if you have temperatures going above zero. If it stays really cold, it's not as big of a deal, but as soon as you hit the temperature where salt can work on ice, it's going to be working on your frame. The hotter it gets the worse it gets.

If you're just using it for your house and tooling around town during a storm, it's not as big of a deal, but I would still recommend rinsing it off whenever you can.

Thought this was worth a separate reply; you also don't necessarily need to bring the washer inside, as long as you make sure there's no residual water in the pump. So you would disconnect the water supply, and then run the pump until nothing else is coming out. It should be fine to store in the cold as long as you do that. At least, that's what I've done when I keep it in the garage. If you're concerned, I would just look that up quickly to see if it's fine to store in. Really cool temperatures as long as you drain the pump.