The roots usually go down about two feet then spread outwards. So between digging a hole or filling a trash can, I'd go with digging a hole. However, digging is a lot of work. If you have a hammer and some nails, go to Home Depot and build a basic 3'x3' raised bed. The people at Home Depot will even measure and cut the wood for you. Then fill it with potting soil. Over here on the west coast I can pick up a 3.5 cu ft bag of Kellogg Patio Plus for $4. Figure $20-$25 in soil should be enough to fill the raised bed. You don't have to do any digging and you won't need to fertilize for a few months, if at all. I have a 7ft Sativa growing in my raised bed right now (Hawaiian Snow by Green House Seeds) and it's significantly taller then the ones I have directly in the ground.
You don't need to spend money on Fox Farm or Roots Organic. You just want good potting soil that has lots of organic stuff in it, like compost, that will break down and feed the plants over time. Your local garden centers, like lowes and home depot, should have everything you need to grow monster size plants.