Although I have crossed words with Andy on a couple of occasions, I must totally agree with him here. (Andy, by the way, Al's comment about field growers was directed at someone else and he had his tongue firmly planted in his cheek).
As for the possibility of purchasing stock for (approx) $50.00 US, it can be done! I have visited several nuseries (Bill Valavanis, New England Bonsai, All Shapes Bonsai to name three) and have always found several trees I would think about in the $50 or under range. Some where in nusery pots, some in mica training pots and on occasion some in ceramic pots. True none were advanced but they were all far better along than anything from Home Depot et al.
At first as a beginner I looked for 'bargains' at every garden center and home improvement store. I may have gotten trees at a more modest price, but they had non compact roots.
I finally started to look at Bonsai Stores/Nuseries for my stock. At first, yes these were smaller trees but they all had a somewhat compact root system and usually had some initial training.
It was amazing (or not so for the knowledgeable) but the amount of tree loss that I experienced was dramatically reduced once I started obtaining my stock from 'better' sources.
Thats my take on the so called $50.00 tree possibility!
Jay