Adding on to my dumbly made support from September It would have to be a really low drop rate so that they remain the same price as creeper heads not much better but resupport
Honestly, I dont support. Because some individuals have spawners in double digits, maybe triple, the rate of spawning mobs are increased in folds for each spawner they place. I put 5 pig zombie spawners - and within 15 mins...i had about 300x Pig Zombies at the grinder location. Imagine if people placed 35. Thats literally about 2100 mobs per 15 mins. it would kill the economy. Then its all about farming, which will make them very rich. The amount of ores in generators is not even low for someone who grinds everyday.
VERY LOW support. only if the rate was like 1% chance maybe even less as even with kill aura which your allowed to use on your own island these could still be farmed reasonably fast. on the plus side may move some spawners out of gold production then.
The drop rate would have to be really rare. I say this because I've been at islands like GrinderIsland where there is an AFK player and sometimes there are 2000 zombie pigman or creepers there. Killing all of them would make it far more likely to get a head and that seems way too easy and would devalue mob heads overall.