One of the main factors that goes towards the price of these Lego kits is the size. Typically, the smaller sets with fewer pieces are less expensive as there isn't as much for kids to spend time building or playing with.
On the other hand, the sets that have more pieces and are larger in scale are priced higher. The kits that have 1,000+ pieces tend to be among the most expensive. The age recommendations for these larger kits is usually raised as they are more of a challenge to assemble.