So I've been doing some math on how much money I'd be spending per year if I kept my current iPhone vs if I sold it and bought a new model, and I'd like some peer reviews to make sure my math is correct. I bought a 128GB iPhone 7 for ~$800 with tax last year. Cost of ownership per year would be: 1 year - $800 2 years - $400 3 years - $267.68 Let's say I sell my phone now for ~$500. That would mean I would have paid ~$300 to use it for one year. If I buy an iPhone 8+ for $860 with tax, that would mean that I would be spending $360 to upgrade. ($860 - $500 I made from selling iPhone 7) Therefore, it would be a better value to sell my iPhone 7 and get an iPhone 8+ than it would for me to keep my iPhone for another year and buy next year's phone brand new. Is this correct?