The decision is usually made emotionally and there is a straightforward way to think about it.
The calculation
Repair cost against the phone’s remaining useful life and its replacement cost.
Remaining useful life is largely determined by software support. A phone with three years of security updates left is worth repairing. One with six months is not, regardless of condition.
Check the manufacturer’s stated support end date. It is published and almost nobody looks at it.
Repairs that are almost always worth it
Battery replacement. Cheap, restores endurance and sometimes performance, and extends usable life by years. The single best value repair there is.
Screen replacement on a phone with substantial support remaining, if the cost is well below a replacement.
Charging port, which is frequently just dirt and free to fix with careful cleaning.
Repairs that usually are not
Anything involving the main board, which typically costs a large fraction of the phone’s value.
Water damage, where repairs frequently fail later even when they initially work.
Multiple simultaneous problems, which indicates the device has had a hard life.
Where to have it done
Manufacturer service is most expensive and preserves water resistance and warranty.
Independent repair is cheaper, and quality varies enormously. Parts matter – a cheap screen is visibly worse.
Self-repair is increasingly viable, with manufacturers now selling parts and publishing guides under regulatory pressure.
The thing worth knowing
A repaired phone with a new battery frequently feels close to new, which people consistently underestimate before trying it.