Need assistance understaning

So in my home country, our phone bills don’t really work like how it works in the US. My mom set me up with a phone plan for an iPhone 15 Pro Max. Also, I offered to pay for my bills starting June and I was thinking of upgrading my 15 to a 16 and I checked the trade in value and it said it was $1000. What does that mean? How much will I pay monthly? Thank you so much for your help!