In my experience the most cost effective serial protocol EEPROM is the 93C46. This provides a 64word by 16 bit memory. It uses a SPI type interface however. Atmel has a whole series of I2C interface serial EEPROMS in a whole range of sizes and corresponding range of prices. I have normally found that these I2C parts are a little more expensive than 93Cxx series parts of similar size. One reason for this cost difference is due to the sheer number of 93C46 and 93C56 used in the market place that are made by many companies. As time marches forward the I2C serial EEPROMS will go lower in price now that one of them is used on each PC type memory DIMM.

Hope this helps
Mike Karas