I cannot tell you the value specifically of slime spawners, but I can tell you some of their problems that makes their value lower. Slime spawners will only work in slime chunks, thus making it a guess and check situation until you find somewhere to put it, but that is not the major problem. The major problem that makes slime spawners much less worthwhile than they would be otherwise is that when the slimes die, either by a machine or by a player, they do not break up. Meaning that you do not get slime balls from the large and medium slimes. This makes slime spawners produce slime balls at a very slow rate, like about 3 stacks a hour if your manually farming it with looting, and only like 40-50 a hour on auto. So realistically slime spawners are not that valuable since they are just so slow. I could not put a exact rate on it, but its probably not overly expensive.