The honest answer is "it depends," but it depends on only a few things. Run your phone through these and the decision usually makes itself.
The one-line rule
If the repair costs less than about a quarter of replacing the phone, and the phone is under roughly three years old, repair it. A $79 battery or a $129 screen on a phone that would cost $1,000–$1,400 to replace is an easy call — you're spending well under 10% to get another year or two. The maths only gets interesting when the repair creeps toward half the replacement cost, or when more than one thing is wrong.
The four questions that decide it
1. How old is it, and is it still getting updates?
A phone still receiving OS and security updates is worth keeping. Once the manufacturer drops support, that's a genuine reason to upgrade regardless of the repair cost.
2. How many things are actually wrong?
One fault — a screen, a battery, a port — is almost always worth fixing. Three faults at once usually means the phone has had a hard life, and the money is better put toward a replacement.
3. What's the repair as a share of a new phone?
Compare the quote to what you'd realistically pay for an equivalent handset — not the headline RRP of the latest flagship. A $149 screen against a $400 mid-ranger is a different decision than against a $1,800 flagship.
4. Do you actually want a new phone?
If you were itching to upgrade anyway, a fault is a fine trigger. If you're happy with what you've got, there's no virtue in spending ten times the repair cost to chase a slightly better camera.
A worked example
A three-year-old iPhone with poor battery health and a small screen crack. Battery: $79. Screen: $149. That's $228 against perhaps $900 to replace with a like-for-like handset — about 25%. Borderline on the rule, but the phone is otherwise healthy and still supported, so most people repair and run it another two years. If that same phone also had a failing camera and a dodgy port, the answer flips toward upgrading.
The bottom line
Repair when it's one fault, the phone's under three years old, and the fix is a fraction of a replacement. Upgrade when faults stack up, support has ended, or you wanted a new phone anyway. Still on the fence? Bring it into Riverwood Plaza or call (02) 9533 3300 for a free, no-pressure assessment.