If I have a broken iPhone how much would it be to trade it in and get a new one?

That depends on the country in which you live.

Look it up:

http://www.apple.com/support/service/

In the U.S

"

Out-of-Warranty Service

If you own an iPhone that is ineligible for warranty service but is eligible for Out-of-Warranty (OOW) Service, Apple will service your iPhone for the Out-of-Warranty Service fee listed below.

A $6.95 shipping fee will be added if service is arranged through Apple and requires shipping. All fees are in U.S. dollars and are subject to local tax. "

US $149 for an iPhone 4...your phone cannot be in pieces.

Apple doesn't do trade ins, though, in some places they work with companies who do. Broken phones don't generaly garner much. You might be better off selling it for parts. Depending on what is wrong with your phone, you may be able to get an out of warranty replacement which would require you turn in the old one.

Just to clarify

The OOW is an exchange program you get back exactly the model you hand in broken, for the price shown by rg

above