SanDisk ( SDSK) is flashing some astounding Black Friday prices on flash memory devices. Flash drive prices have fallen faster than those of hard drives. USB thumb drives now pack three times the memory capacity at a third of last year's prices. The numbers. The SanDisk 4 gigabyte flash drive is available for $8 at Wal-Mart, a 73% discount from the $30 regular price. In contrast, Target ( TGT) sells a Lexar 4 gigabyte flash drive for $20. The stock. Falling memory prices haven't hurt SanDisk shares. The stock has doubled this year. A big part of that enthusiasm, however, was due to a positive resolution to a licensing dispute with Samsung in May.