Proportional transaction cost
= Proportional transaction cost
{title2=$\varepsilon|\theta|$}
A holding-dependent cost proportional to the absolute position size. In a one-period gain objective it changes the sample payoff to $\theta X-\varepsilon|\theta|$, giving gain parameter $X-\varepsilon$ for positive holdings and $X+\varepsilon$ for negative holdings. The objective is concave but can have a kink at zero.