Minimum cost topological ordering
Your problem is NP-hard. I show this by a reduction from the shuffle problem: given words $w, w_1, \ldots, w_k$ over the alphabet $\{a, b, c\}$, decide whether $w$ can be obtained as an interleaving (aka "shuffle") of $w_1, \ldots, w_n$. This problem is NP-hard: see Warmuth &amp; Haussler, On the complexity of iterated shuffle, JCSS, 1984, Theorem 3.1.

Given an instance $w, w_1, \ldots, w_n$ of this problem, and writing $l_i := |w_i|$ for all $1 \leq i \leq n$, we build the DAG $G$ as a union of path graphs $L_1, \ldots, L_n$, where each $L_i$ for $1 \leq i \leq n$ has $l_i$ vertices written $v^i_1, \ldots, v^i_{l_i}$. Now, we define the cost function $f$ as follows: for each $1 \leq i \leq n$ and $1 \leq j \leq l_i$, for each $1 \leq k \leq |w|$, we set $f(v^i_j, k)$ to be $0$ if the $j$-th character of $w_i$ is the same as the $k$-th character of $w$, and $1$ otherwise.

This reduction is clearly in PTIME, and it is clear that the minimum cost of a topological sort is 0 iff there is an interleaving of the path graphs realizing exactly the word $w$, showing that the reduction is correct.

(Shameless ad: If you're interested in NP-hard variants of topological sorting, you may be interested about a recent preprint of mine which studies the complexity of finding topological sorts that fall into fixed regular languages.)