Device Upgrade

I will start by saying that the only reason I have 2 mobile lines is because a while back when I decided to drop cable and only have internet, I was told by the agent that I could save a good deal of money if I added an additional mobile line.  Made zero sense to me because how is adding an additional mobile line and sending me a phone going to save money but, sure, whatever.  

Fast forward, I would like to allow my son to use this 2nd line.  They sent me some Motorola phone when they convinced me to have the 2nd line for this "cost savings".  I would like to upgrade that to an iPhone 14.  I tried to do that today, and it shows the cost would be 9.58 a month.  Cool.  Until I choose that I want to upgrade the line with the Motorola, then the cost goes up to $24 a month for the device.  

I tried a chat and basically was told too bad.  Your Motorola is not trade in worthy so you have to pay the full price.  So really, I'm not "upgrading", I'm more or less just buying a new phone.  Just say that and don't act like it's going to be a deal, because it really isn't.  For $9.58 I absolutely would do it AND up that line from a 1GB line to an unlimited line.  For close to $26 a month, no thanks.  I am pretty frustrated, as I usually am when I deal with anything Xfinity related.