Passive mobs (pigs, cows) can only spawn on grass, so make sure you build a preferred 10x10 platform with fences around it so the mob doesn't fall out. Sell the loot the mob drops too, so that you can always make that extra profit. Spawners can stack , and you can upgrade them by placing a spawner on top of a spawner and they will merge. It'll take you a while to get through all the stages , so preferably you should get atleast x5 of spawners of the mob.