Straight from the ground versus established in a pot means something to most people from a price/value standpoint. A percentage of freshly dug trees don't live so there is a gamble involved and the price generally reflects that. Whereas, the tree you dug and grew for 2 seasons is now a known commodity and less risky; plus you now have 2 seasons of time and materials invested in it. In my opinion, you will make more money by digging trees and spending a couple of years developing them. You can sell them as established trees and maybe even initially styled/wired etc and that will raise your price. On the other hand, if you have access to a lot of trees, then maybe you want to focus on volume and sell them freshly dug for lower cost. My advice is just do it. Pick a spot and start. The market will correct your pricing. You will soon know if your prices are correct. Sell good trees for good prices and demand will run you ragged. Sell bad trees for the wrong price and you will not be selling for long. If I understand your set up, you have access to material for free and all you have to do is dig. What's that worth? Your time. Sell auction style and let the market establish the price, then start selling for fixed prices based on your accumulated knowledge and experience. Odds are that unless you enjoy that process you will discover your time is better spent doing something else.
The OP asked about pricing trees. Well, there is no magical formula that answers the "what's it worth" question. There are many variables that go into pricing. Supply and demand are the obvious starting points, but not the whole picture. Bonsai is a niche market, and high end trees or certain species are niches of the niche, so supply and demand in a timed market don't really tell you value/worth. Age, species, potential, development, pedigree, flaws, location, health, are a few significant factors to consider. Most people consider bonsai trees luxuries and not necessities, so there is no "need" to buy. It comes down to choice, and that means personal preference. To someone, that Brussels' ficus is worth $5k or $10k or $25k or whatever amount they are willing to spend. That is their own personal preference.
My advice is simple. Research, take your time, find what you like, and buy the best you can afford.