Market prices are regional and would be influenced, I believe, by the added value its use brings. Consider 2 scenarios. 1) an apple orchard producing at 50% of its genetic potential and 2) a rice paddy producing at 50% of its genetic potential. The orchard is currently grossing about $10,000 per hectare and year, while the rice paddy is grossing about $500 per year and hectare. Let's simplistically assume that 10 tonnes of biochar per hectare in each scenario would increase yields to 100% of their potential. The amount per tonne the rice farmer would be willing to pay would be less than the apple farmer, because a $500 gain in gross yield would be less than a $10,000 gain in gross yield.
Agricultural realities, and the perception of value, are of course more complex than that, and not all chars produced as a byproduct of a pyrolysis process will be optimal for agricultural fertility. In a word, it's complicated, but we are constrained by the way evolutionary processes have "designed" soil fertility in the case of agricultural plants. That's what counts. In my simple view, to optimize soil fertility, the end product of pyrolysis must have a molecular structure as close as possible to that of humic matter, or char produced from wild fires.