The price of a bow is determined by the material and quality of craftsmanship. Certain materials are very expensive, particularly really good pernambuco, since it is endangered and export out of Brazil is heavily restricted.
How much time and care the maker puts into the bow is another big factor. The person making the bow will spend the most time and do his best work on his finest pernambuco stick, and usually embellish it with expensive materials like silver or gold. This makes a really good bow really expensive, since it is an intersection of the best materials and the best workmanship.