Encuentra intervalos de fechas con mysql

Esto debería dar el resultado correcto:

select
  id,
  min(startDate) as startFreeDate,
  count(*) - (endDate is null) numFreeDays
from (
  select
    pb1.id,
    pb1.bookingDate startDate,
    min(pb2.bookingDate) endDate
  from
    pricesBookings pb1 left join pricesBookings pb2
    on pb1.id=pb2.id
       and pb2.price>0
       and pb2.bookingDate>pb1.bookingDate
  where
    pb1.price=0
  group by
    pb1.id,
    pb1.bookingDate
) s
group by id, endDate
order by id, startDate

véalo aquí .

Si necesita buscar todos los espacios libres de, por ejemplo, 14 días, puede agregar HAVING:

group by id, endDate
having count(*) - (endDate is null) >= 14
order by id, startDate