Do you really not see how ridiculous this speculation is? No one can tell you exactly what you will get on tradein or private sale. It depends on the used market when you go to sell, what methods of selling you are willing to try, and the current condition of the machine.
If you needed 64 GB of ram or these other upgrades, you would already know it.
The buy and return cycle just suggests you drastically overestimate your own requirements and have no idea how to determine how much you'll lose later. As others have suggested though, spending less up front means you limit potential losses in 2-3 years. You should of course not expect it to retain the majority of its value, given that specs change and all machines have finite lifespans. If you instead assume that you'll get nothing on resale later, you'll at least be honest with yourself as to how much you are willing to pay for the use of the machine.
