Spawners for some oneblock servers (atlantis/forest/space) cost $1,000,000 each. Most spawners in their current state have no value. The only spawners with actual value would be iron golem and vindicator. Others such as witch, blaze, sheep, etc. could have some value for giving you essential building blocks or higher XP. However there are many like skeleton, creeper, spider, enderman, etc. that do not really have value on oneblock, to my knowledge you can’t sell gunpowder, bones, spider eyes, string, enderpearls, etc. which give the spawner in return no value. If some sort of reasonable value was given for these items it would give these spawners an actual purpose other than just XP farming.