Use this phrase when you're telling people about something they don't know about. If you know some information that you think they won't expect, you say "You'd be surprised how ___ it is to ___". For example:
You'd be surprised how expensive it is to put your kids through private school these days.
You'd be surprised how easy it is to get help with your job search if you just ask around.
There are also some shorter versions of this. When someone makes a guess about something that you know about, and their guess is wrong, you can say, "You'd be surprised."
A: There aren't that many people in Seoul who speak English, are there?
B: Oh, you'd be surprised.
"You'd be surprised to...." is usually used in spoken English, not often in writing.