We could not possibly give you an estimate here. Screen cracks vary in severity widely.
You might get an idea by a two-step approach:
- Figure the price of repairing a cracked screen: iPhone Service Pricing - Apple Support
- Get an estimate of the iPhone 7 Plus value: Renew and Recycling - Apple
So naturally you would subtract the cost of repairing the cracked screen from the iPhone 7 Plus value to get your credit amount idea. Then you would add the price of the new iPhone you wish to purchase.