For every plant, I like to devote 100 watts -and I don't care what kind of efficiency I get from the light I use because I usually only grow about 4-5 plants (I only grow for my own use -never sell) so efficiency differences come out to negligible differences in cost -something that can easily be deferred by abstaining from a couple of dine-out meals or a few six packs of beer per year.
I like to use tent to grow in because it's something that can be taken down and put away when/if I might need the space for other things at other times of the year. My 3 X 3 tent cost me about 225 dollars. A charcoal filter and fan are a must...add another 200...need some oscillating fans...add another 100...need pots, dirt, fert, misc odds and ends...200
So, the quick breakdown is approx 500-700 in lights (depending on what kind of lighting you prefer...I use COBs atm)...and another 500-700 in other 'stuff'....
And that's for approx. 5 plants grown the way I grow them in dirt. If you want 10-15 plants, then you can multiply from there. If you want a hydro setup or some other style of growing, then idk anything about that stuff.
You could easily take a less expensive DIY route and get more for your money (depending on how much you charge yourself for your own time to make the stuff), but I opt for pre-fab and getting the best equipment I can afford.
For a quality 4-5 plant indoor setup, I estimate approx. 1000-1500