This essay argues that late nineteenth-century American fiction, as exemplified by William Dean Howells’s The Rise of Silas Lapham (1885) and Theodore Dreiser’s Sister Carrie (1900), is entangled in the period’s embrace of a new understanding of economic conditions, one premised on the significance of consumer data as the raw material of economic interpretation and fixated on mathematical analysis as a source of knowledge about value. Marginalism in economic theory and realism in American literary practice were contemporaneous efforts to mediate value at a moment of decisive transition in America, as a model of commodity-centered competitive capitalism yielded to the regime of corporate finance. The young academic economists of the period and their literary counterparts explained the economic landscape in much the same way and to many of the same ends. Both began with the daunting question of how to reimagine value in a world characterized by fictitious goods and increasingly abstract financial instruments, a world in which the labor theory of value seemed hopelessly outdated. Both responded to this challenge by proposing novel intellectual procedures for making values appear stable again by focusing on the subjective experiences of consumers. Finally, both marginalists and realists sought to provide a quasi-scientific bedrock for fixing values in consumer society not only by appealing nostalgically to an economy of actual goods and honest work but more importantly by accommodating readers, policy makers, and stakeholders of all kinds to the new realities of finance capitalism.