I don´t agree that potential advantage in the exchange comes ever from
significant differences in piece values, and good examples comes from
positional games like Xian-Qi or Hexetera/Etcetera. In Hexetera, my
subjective estimation of values are, fixing Pawn in 1: Man 1.5,
Flyer-Elephant 2.5, Guardian 4, Rook 5.5; but in this game the usual
exchanges for advantage are strictly positional, and many times (really
many times)this kind of exchange is performed exchanging a major piece for
the capture a piece of less value, i.e., conceeding material. In this game
there is not permissed to change pieces of the same type, making this game
almost estrictly positional, and sacrifices are not only usual, but many
times necessary for a definition, finishing a game in around 40 moves. In
Xiang-Qi, material advantage is not as important as positional advantage,
and other of my games, Deneb, is clearly a very positional game, being
that all the major pieces have approximately the same medium value, around
a little less than a FIDE-Rook, but the extinction rules induce games that
lasts in average 25-35 moves. It is difficult establish good measures for
positional games in which material advantages are not determinant. I´ll be
back with other games in which good measures are not easy to stablish
properly.