Applying salt to slugs dissolves them, and is cheaper than pellets. Is this humane, or are there preferable techniques?
Not sure this is the right place for this question, probably would have been best in the Garden and Landscape section, but there is a slightly hacky answer:- personally, I prefer the beer trap solution - you fill a small container with some beer, sink it into the soil (you'll need more than one for a large area), the slugs go in and eventually die. But at least, hopefully, they won't know much about it because they're drunk...
Salt does actually kill a slug quite quickly, but I cannot imagine its a comfortable process, being dehydrated so quickly. Salt, though, should not be applied to the soil, so the slugs need to be put into salt rather than it being sprinkled onto the soil where they are.
Other alternatives are nematode applications which target slugs, yet don't harm anything from soil to other creatures, but they aren't a cheap solution.
You can certainly reduce the incidence of slug and snail damage by using things like copper bands and gritty or drying materials around plants, or encouraging particular forms of wildlife which will eat them - more info on that here