How much would it cost for me to go from my free 16GB iPhone 6 upgrade to a 64GB iPhone 6?

I currently have an iPhone 4, but I'm able to have a free upgrade to a 16BG iPhone 6. But what I really need is a 64GB iPhone 6.

How much would it cost for me to upgrade my upgrade?

No more then a hundred bucks considering that its the same phone and model just more space capacity