Ye gods, some kid's outdoor clothing can be so expensive, especially when they outgrow it or trash it so quickly.
One of my techniques is to shop for sale merchandise at REI, or on Steepandcheap. I recently got her a larger REI down jacket for $40, and a Marmot rain coat for about $30. In the past I've gotten good deals on rain and snow boots, convertible nylon pants (we call them "adventure pants"), winter hats, jackets, gloves, shoes and socks.
It takes some time and it helps if you can be flexible about timing your purchases, but you can save a lot of money on decent quality outdoor clothing.