Pricing in Supply and Demand is decided by Bargaining Power, not the other way around.
The idea that workers 'outnumber' the capitalists does not give them power, it's the complete opposite.
I have no idea why you would think think they have some kind of power in that equation especially when there is probably a labour surplus.
If workers merged and formed a coherent Union, and acted as a single monolithic entity i.e. 'all labour in the colonies comes from a single Guild', then they'd possibly have more power, but this wouldn't really be possible obviously.
The entire treatise is essentially about the disproportionate power of concentrated wealth and that's what will ultimately drive wages.
That it's difficult to quantify doesn't make it false.
'Profit concentration' is literally a measure of value chain power: if you can collect a lot of profit, it's a sign that you have leverage over the other entities: workers, financiers, customers and suppliers.
Apple has incredible leverage over vendors, which is why even if you sell some really cool and awesome component to them for their iPhone you might not make Jack in profits, and may find yourself out of business a few years later.
Apple makes 100x more margin per device than Foxxcon, which illustrates the capital vs. labour dynamic quite well.