Estimate solo travel costs.
Solo travel costs more per person than the same trip shared, and accommodation is where the difference lives. A room priced for two occupants is charged to one, and organised tours add an explicit single supplement on top — commonly 15% to 40%. Flights, food and entrance fees are per-person costs either way and so carry no penalty. Isolating the supplement makes the avoidable part visible: hostels, guesthouses and properties that price per person rather than per room remove most of it.
Solo travel cost
Lodging = room rate x nights x (1 + single supplement); total = flights + lodging + daily costs x nights + activities
Book accommodation that prices per person — hostels, guesthouses and some Japanese and Nordic hotels do. On organised tours, ask about room-sharing schemes, which many operators run specifically to waive the supplement.
Per day, yes, mainly through accommodation and any taxi you would otherwise split. Against that, a solo traveller controls every spending decision, which in practice often offsets a good part of the gap.
Slightly less in absolute terms — no shared plates or rounds — but street food and market meals shift the figure far more than travelling alone does. Set the daily figure from the destination rather than the party size.