my initial guess was that it was more humiliating for her mother to be cordial (and thus would deal a heavier blow), seeing as she was the one who threw the first stone and caused this situation in the long run.
not to mention, it did seem like the item was not as special to mc and the male lead (since he apparently has a lot of them hidden), so a fair price would be to effectively damage her mother's price, but not too much that the other party would back down from the deal.
I guess its to bring her mother down a peg or two. Someone with that much power and pride greeting someone as an equal when they ised to see them as insignificant is a heavy blow to their ego.
Her mothers greeting seems like THE MOST UNNECESSARY AND USELESS TRADE OFF.