I have done two grows so far this year. I ended up with somewhere around 950 grams dry total. Im guessing I have about $1200-$1500 invested in those two grows including start up costs for lights, tent, seeds, nutes, tools, etc etc etc.
Around here, the cheapest you can buy any dried bud is $10/gm, so I have grown a minimum of $9500 worth of pot for an investment of $1500. That means my growing has "saved" me at least $8000.
Thats where I stop when Im talking to my wife.
The part I leave out is that I only use about 40 grams/month. That means its going to take me about two YEARS to use up all that pot. I was spending about $200/month before I started growing, but my usage has doubled now that I can afford to take as much as I need instead of being limited by what I could afford.
Once you get past the initial investment, the on-going costs are not that much. My electric cost me an extra $35 or so a month. Nutes and other supplies seem to be running around an additional $50-$75/grow, but I also spend a good bit on making changes to the grow - like switching to hydro, and then changing again to HPA.
Still, if you are buying bud now, growing will (maybe) save you a good bit.