If you think of mushrooms as a cycle, you can "buy in" right before they produce the mushroom bodies from pre-colonised blocks of grain. You're correct, at this point all the hard work has been done and the purpose of the 'container' is to build up humidity and co2, it can even be as simple as a plastic bin bag. This is the quick, easy and expensive way in my understanding, you make it cheap and time consuming by creating, sterilizing and inoculating those containers of grain yourself. It's worth doing if you plan on growing more, but if you just want to get started then buy the pre-sterilised or pre-colonised grain.