Violin family instruments have tops, backs and ribs that are fairly thin. These plates expand and contract with weather changes. If they are subjected to sudden humidity or temperature changes and the plates can not gradually adjust to their new size, cracks can develop. If you develop cracks that are adjacent to the saddle, most likely it's a sign that the saddle is a little too tight and it's not allowing the top to contract.
If your instrument develops a crack, it should be repaired as soon as possible. Be careful not to touch the crack edges with your fingers or get any dirt, rosin dust or polish into the crack.
Many cracks can be glued from the outside, especially if it's in an area without too much stress. One of the seams may need to be unglued before the crack is repaired. Sound post cracks or bass bar cracks usually need to have the top removed and patched.
Violin Repair: It's somewhat difficult to give prices without seeing the instrument. It it's possible to glue it from the outside the range could be between $12 to $40. If the top needs to be removed it's about $300 and up for a violin.