A method for operating a data storage device including a non-volatile memory device including a first region and a second region includes: storing data from a data cache memory in memory blocks in the first region; determining a first garbage collection cost with respect to a first target memory block having the least valid page among the memory blocks in the first region in which the data are kept; determining a second garbage collection cost with respect to a second target memory block having the least valid page among the memory blocks in the first region from which the data are cleared; and performing a garbage collection operation to copy valid data of a garbage collection target memory block into memory blocks in the second region based on a comparison result of the first garbage collection cost and the second garbage collection cost.