pantera is probably not the best example to use to gauge another comparable bands worth. they obviously blew ridiculous amounts of money on partying and destroying things. also factor in damageplan was surely a large step down in the money they were bringing in, but probably were blowing money as if they were still pantera.
granted i dont know much about their lifestyles, but slayer strike me as pretty tame and unextravagant in comparison. not to mention they have been at it far longer and have been relatively big the majority of that time. if there is one underground metal band that is financially set for quite some time its probably them.