In most regions the Trade-In value will be refunded to the payment method used after the device has been purchased. It will not be applied directly to the purchase price at all.
The gift card however would apply directly to the purchase price including taxes during checkout.
So basically you would be paying in your example: $1150 (price and taxes) - $300 = $850.
Then you would get a $200 refund to the payment method for the Trade-In.
How does Apple Trade In work?
Important: If you are currently enrolled in the iPhone Upgrade Program, please upgrade your device through that program and do not proceed with Apple Trade In. Check your upgrade eligibility status
1. See what your smartphone is worth.
Answer a few questions to get your estimated trade-in value. If you pay monthly, we’ll apply the value as an instant credit to lower the monthly payments of your new iPhone. If you pay in full, we’ll credit your payment method after we receive your trade-in.