I would suggest that you look at the Dallas/Maxim part number DS1672. This part is quite low cost and has I2C serial inteface. The reason this chip is low cost is becasue it operates a little different from the conventional RTC chip. Most RTC chips have a series of specialized registers that count the YY/MM/DD HH/MM/SS values.
The DS1672 has a simply a 32 bit counter that counts seconds. A software algorithm is used to convert the seconds count to the normal time component values. There are three APP notes on the Dallas web site for this part. App Note #31 describes the algorithm necessary. App Note #511 shows use of this part with an 8051 derivative and accompanying sample software in C code. The third App Note #95 shows the algorithms in 8051 assembler language.
Good Luck
Michael Karas