Flash memory is widely used as storage and portable devices such as digital cameras, MP3 players, cellular phones and laptop computers due to its characteristics: large volume, non-volatility, low power consumption, and good performance. However, a block in flash memory should be erased to write data because of its hardware characteristic called erase-before-write architecture. Erase operation is much slower than read or write operation. Hence, we can use FTL to overcome this problem without any modifications of upper file system. Many DBMSs execute some queries based on diskbased cost model. But if flash memory replaces disk as a storage device, disk-based cost model for query optimization cannot be used at flash-based databases. We propose flash-based cost model and compare it to disk-based one.