The economics of private tours change once you stop comparing per-person sticker prices in isolation. A couple sharing a private vehicle and guide at 3,500 MAD per day is paying 1,750 MAD each — not far above many small-group rates, and for a completely different product: your schedule, your pace, your choice of where to stop for lunch.
The bigger advantage is less visible: a private guide builds rapport fast. By day two they know whether you want to photograph everything or move quickly, whether you enjoy the long explanations of Berber architecture or prefer to wander on your own. A group tour guide cannot do that — they are managing a bus.
If budget is the primary constraint, a reputable small-group operator is a legitimate second choice. The key is still the same: licence, named guide, written itinerary, decent reviews. A private guided tour simply makes those qualities easier to verify and to hold the operator to.