Cost?

There is no charge to use Family Sharing but using Apple Music with Family Sharing does require a higher-priced family plan.

More information:

Family Sharing - Apple Support

and

Apple Music and Family Sharing - Apple Support

and

Manage your Apple Music membership on your iPhone, iPad, iPod touch, Mac, PC, or Apple TV - Apple Support

That can be accomplished without using Family Sharing. The following has more details: If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

Note that it can be done from any computer and can also be done from any iOS device which has installed the Find My iPhone app. https://itunes.apple.com/us/app/find-my-iphone/id376101648

You don't need Family Sharing for that at all.

You can use the Find My iPhone App on any device (including your friend's devices) and even on http://icloud.com to locate your iPhone if its lost.

http://www.apple.com/lae/icloud/find-my-iphone.html

You can use any computer that has internet access -- it doesn't have to be an Apple product.

and

You can have your friend install the Find My iPhone app on his phone. If your phone is lost, you could sign in to your Apple ID in that app on his phone in order to locate/lock your phone.