At the beginning of "The Luncheon," the narrator mentions that he was short of money at the time and had only eighty francs to last for the remainder of the month. It is therefore clear that he will be uncomfortable whenever the lady requests anything expensive to eat. He is relieved by her assurance that she never eats anything or more than one thing for luncheon, but his heart sinks each time she mentions some rare delicacy.
First, the salmon is an item that does not even appear on the menu, meaning that the narrator cannot budget for it. In contrast to this is the caviar, which he knows he cannot afford. He shows his discomfiture to the reader by ordering the cheapest dish on the menu, a mutton chop, but the lady does not take the hint, continually remarking that this cheap, simple dish is too heavy for luncheon. These remarks make it clear that she has missed the point of the narrator ordering something cheap and is completely indifferent to how much of his money she spends. Throughout the story, she attributes his refusal of all the delicacies she enjoys to his eating too much mutton.
W. Somerset Maugham also shows the growing discomfort of the narrator both in his increasing antipathy to the lady and by his obsessive focus first on how much of his eighty francs the meal will cost, then on whether he will have anything left, and finally on whether he will be able to pay the bill at all. Topics that would usually interest him, including his own work as a writer, cease to have any interest for him as he calculates the ruinous effects of his companion's extravagance.