Suppose you always want to be running the latest iOS release, but you want to replace your phone as infrequently as possible. You would "only" have to have purchased 4 iPhones since 2007:
| Model | Launch date | Obsoleted by | Price
|-----------|--------------------|--------------|------
| iPhone | June 29, 2007 | iOS 4 | $399 (*price cut)
| iPhone 4 | June 24, 2010 | iOS 8 | $599
| iPhone 6 | September 19, 2014 | iOS 13 | $649
| iPhone 11 | September 13, 2019 | - | $699
Adjusted for inflation, the total for these phones is $3,287 excluding carrier contracts. Assuming the iPhone 11 will be obsoleted by iOS 27 in September 2026, this costs you about $14.29/mo.