Where a PhD is effectively required
Buy-side quantitative research at the major systematic managers is the clearest case. The work is original statistical research, the peer group is doctoral, and the hiring process assumes you can present and defend your own research. Entering without a doctorate is possible but unusual and generally requires equivalent published or professional research.
Where it is not
Quant trading is hired on reasoning speed and calibration, and firms actively prefer to train from an undergraduate base. Quant development and low-latency engineering are hired on engineering depth; a PhD is neutral at best and can be a mild negative if it came at the expense of production experience.
Portfolio-manager seats are hired purely on track record. No credential substitutes for attributable profit and loss.
The honest cost calculation
A PhD takes four to six years during which an undergraduate-entry trader may already be earning a substantial bonus. If research is the goal, the doctorate pays for itself and is the only realistic route. If the goal is simply to work in quant finance at a high compensation level, it is an expensive detour.