1. A garbage collection method of a memory system including an interface device, the method comprising: applying, by the interface device, a weight to a first factor and a second factor to calculate garbage collection costs, the weight for the first factor being higher than the weight for the second factor; configuring, by the interface device, a hash table using the calculated garbage collection costs; searching, by the interface device, a block having the lowest garbage collection cost from the hash table; and performing, by the interface device, garbage collection on the searched block, wherein the first factor is wear level information and the second factor is information of a valid page number per block or the second factor is wear level information and the first factor is information of a valid page number per block, and the garbage collection costs are calculated by adding at least a product of the first factor and the first weight and a product of the second factor and the second weight, with respect to each physical block.