I've read up a bit on it and yeah there seems to be an exact science if your starting a colony from scratch but I read a post from a guy and he was stating that it could be done easily and on the cheap. He lived next door to a major mushroom farm and was buying their spent compost that was no longer considered productive enough for a commercial operation. He was buying bags of UNTREATED Live compost and was getting a few small runs out of the cheap compost. From what I remember he was paying something like $3 a bag and getting maybe half the number of mushrooms that you'd expect from a $18 kit you can buy from nurseries.
I'll try out his cheapo method as it won't be a too bigger undertaking if it all goes belly up
i see what you mean now. you could also try and mix in some fresh newly pasteurized(but cooled)manure/straw substrate to get better flushes. and correct me if im wrong but dont the grocery store type mushrooms require some sort of casing?
you should read...
The Mushroom Cultivator: A Practical Guide to Growing Mushrooms at Home
http://www.amazon.com/Mushroom-Cultivator-Practical-Growing-Mushrooms/dp/0961079800
this book is great covers about everything you can think of my only gripe is that like half the book is dedicated to makeing mushroom compost, which i was not interested in doing.
Paul Staments also has a speach on ted talks, it was pretty interesting.
i think this book had sections on old school mushroom cultivation in france.
they would basically take a chunk of colonized substrate from a previous run, and toss it into fresh mushroom compost. it takes a long time, but it works.
a better faster alternative that i would your way up to is culturing mycellium in a liquid medium. then inject the mycellium in somewhat large volumes into your pasteurized substrate. you should colonize way faster that way.
ive seen people take an isolated strain of oystester mushroom and grow it out on large disposable petri dishes, then scrape them into a sterile blender, and blend the mycellium into like 2 liters of sterile water. they would then innoculate like 100 bags of substrate with that 2 liters.