I imagine the cost of using passenger planes as cargo planes is at least 4x more than dedicated cargo plane.
Probably at least 70% less storage capacity. Probably can only handle smaller packages, probably not as easy to load/unload, and weight distribution without passengers might even make it so the full (available) space cannot be used.
However, it probably has similar fuel requirements (slightly reduced from less load), and still requires a pilot to transport a much less valuable trip.. making the fixed costs still very high
Most passenger airplanes also takes cargo containers (of a size specific to a group of plane types) below the passengers. Not that many of course, compared to a cargo plane. E.g. even a A321 takes 10 LD3-45 containers.
Depending on the density of what's being transported the lower volume might or might not matter. E.g. the a321, according to Wikipedia, has a max payload 25t (I assume with reduced range). That's not that hard to fill with 10 containers of 3.7m3 each.
I'd assume that one significant reason the flights are that cheap is that they have contractual cargo obligations requiring them to fly some flights anyway. A handful of additional passengers won't reduce the payload capacity meaningfully, but will still bring in more than the increased fuel/service costs.
"An unprecedented collapse in passenger demand is prompting airlines to use their fleets to transport more cargo, including medicines"
shouldn't this, if anything, bring down the cost of chatering a plane?