Hi
The battery in iPhone is designed to retain up to 80% of its original capacity at 500 complete charge cycles.
One charge cycle is completed each time you have used (discharged) an amount that equals 100% of battery capacity. One charging cycle would be completed, for example, after:
- Two charges to full, each starting from 50%.
- Or one charge to full from a starting level of 80% plus one charge to full starting from 20%.
More information:
https://www.apple.com/batteries/why-lithium-ion/
For an iPhone that is out of warranty, Apple offers battery service. Costs vary by region. In the US, for example, the cost is currently $79 plus tax and, if required, shipping:
iPhone Service Pricing - Apple Support