Trade Cost Factor may be implemented incorrectly in this mod.

The Vanilla rules of trading are:

If an item has a market value of 100:

sales tax is x60 (40% loss of value)
New price - 60.

mark-up is x1.40 (40% extra cost)
New price - 140

Social skill bonus alters this @1.5% per level. To make this simpler, we'll fudge it and give the pawn a 10% bonus - impossible in-game.

Selling gains an additional 10% of the new price
Final price - 66

Buying gives a discount of 10% of the new price.
Final price - 126

Regarding TCF:

Assuming "not available when language not learned" is set, we only deal with "dialect not learned" penalty, which is set at x67. I believed that this penalty is meant to replace the default sales tax value of x60, but is actually applied after the final price is calculated, resulting in a second 43% sales tax or price markup!

Selling: 66 - dialect penalty (66 - x0.67 = 21.8) = 44
Buying: 126 - dialect penalty 126 + x0.67 = 41.58) = 167

Social skill has no impact on this altered value either.

I believe lowering the TCF penaty would be the easiest option here. For example, setting the penalty value at x0.90 (10% penalty, similar to using workstations outside, and is practically the same as changing the vanilla value from x0.60 to x0.70) would result in

Selling: 66 - dialect penalty (66 - x0.90 = 6.6) = 59.4
Buying: 126 - dialect penalty 126 + x0.90 = 12.6) = 113.4

EDIT

This can be achieved by setting the "trade cost factor when dialect not learned" slider to 110%. Apparently 100% is actually no adjustment to the final price, and I was just reading it wrong. Or it's not very intuitive? 110% is actually just +10%