Spansion and Winbond have SPI Flashs in every laptop, desktop motherboard, graphics card and almost every Android Tablet ever made in the world, huge huge volume production allows them to offer those low prices, the other difference is the power consumption as already noted.
For a battery powered gadget I opted for a (much) smaller EEPROM, lower power consumption in sleep, read and erases, but I traded 64Mb of memory to 512Kb, so theres that..