for example it would only increase the price by about 3 dollars.
~$1.45 for a ATtiny2313 for the I2C interface and ~$0.75 for a DIP switch to select the address.
It would increase the COST by about $3... (oops. The Lady already pointed this out!)
Isn't implementing an I2C "slave" difficult, though? Or was that SPI? There are a couple protocols where the master has it easy, but the slave has to be able to toggle a signal on a clock edge (or similar), which is tough to do in SW.