It wouldn't be cheaper than buying bulk carbon and not all carbon is equal, some believe there is good carbon and there is bad carbon. Good carbon is awesome, bad carbon kills everything. What kind of carbon is in the home depot filter?
I assume this is what you're referring to:
http://www.homedepot.ca/product/filt...-500361/923082
3500ml of Rox carbon weights about 1.4 kg and costs under $40, many other types of carbon is available for half that.