No your absolutely right, you can control one as a slave. The reason the LCD kit from modern device costs so much, is cause its already programed to be a slave and has all the helper functions programmed into it. you are going to have to program all these yourself. Its not hard at all, you just got to take that into consideration.
If you don't mind doing some soldering, I'd get one of the chips from here: http://phanderson.com/lcd106/lcd107.html Its the chip used in the Modern Device LCD driver. Its pre programmed and you just need to either breadboard it, or solder it onto a protoboard. There was another company that sold an I2C version as well, but I can't find it.