Making attacks on password hashes less economical
I had been thinking of this question for quite a time, but without a satisfying answer: it seemed to be a problem that had never really been considered in the literature (at least, not in the theoretical cryptography community).

Incidentally, I was just checking the ePrint archive this morning, and stumbled upon this paper which was added today to the archive, which focuses exactly on your question. It introduces a new complexity measure that accounts for the fact that time-memory trade-off do not scale linearly with respect to the hardware cost. I've not read it in details yet (I plan to), but the authors are among the leading researchers on memory-hard functions (subsets of the authors developed the theoretical model for analyzing MHFs, described the best known attacks on Argon and Balloon, gave new constructions, and proved the maximal memory hardness of Scrypt). It should be worth reading.