1. A computer-implemented method for estimating transaction costs of a security trade execution according to a trading strategy selected by a user, comprising the steps of: receiving over a network, data defining parameters of a proposed trade execution from a user, and data specifying a user-selected trading strategy, said trading strategy data including a sequence of share quantities of securities to be traded per time interval for a given trading horizon; calculating, using a computer, first estimated transaction costs for the received proposed trade execution based on the user-selected trading strategy and market data using a first agency cost estimation model that considers discretionary and non-discretionary trades; calculating, using a computer, second estimated transaction costs for the received proposed trade execution based on the user-selected trading strategy and market data using a second agency cost estimation model that considers only non-discretionary trades; and displaying to the user at least one of the first and second estimated transaction costs; wherein, said user-selected trading strategy is selected from among a plurality of predefined trading styles, or specifically defined by said user.