Estimate bed and breakfast costs.
The defining feature of a bed and breakfast is whether the breakfast is actually included, and the answer changes the comparison against a hotel materially: two guests over three nights at fifteen each is ninety, which is most of a night's room rate. City or tourist tax is the other line that catches people, because it is levied per person per night rather than per room and is usually collected in cash on departure — so it never appears in the booking total at all.
Bed and breakfast total
Total = room rate x nights + (breakfast per person x guests x nights if not included) + city tax x guests x nights
Usually not. Most European city taxes are collected by the property on arrival or departure and are excluded from the online total, which is why they are entered separately here.
Rooms are normally priced per room, but breakfast and city tax are per person — which is why a solo traveller pays a higher effective rate than half of a couple's.
Once breakfast is included, often yes for two people, because a hotel's breakfast is typically charged separately at a higher price. For a solo traveller the gap narrows considerably.