It varies with market conditions and number of months since release. Example, my iPhone 12 Pro will get me $670 plus tax back, and I paid $999... This past March, Apple paid me $500 for the 2019 iPhone 11 Pro Max which I paid $1099 for...
If you go to Apple's site, they will tell you what they are currently offering. Yes, they credit you back the trade-in price plus sales tax - to your original payment method.