Use a sub query similar to the below.
UPDATE P
SET extrasPrice = sub.TotalPrice from
BookingPitches p
inner join
(Select PitchID, Sum(Price) TotalPrice
from dbo.BookingPitchExtras
Where [Required] = 1
Group by Pitchid
) as Sub
on p.Id = e.PitchId
where p.BookingId = 1