Very simple concept. Why? Well, spawners are the main reason imo for the economical collapse, because they are the source of infinite resources. Most ppl will be against this, because its convenient and easy for xp, but that doesnt make it right. If it keeps getting abused the situation will only get worse. My idea is replacing every mob spawner with eggs, but for lower prices of course. Eggs like zombies or skeletons arent really necessary at all, but passive mob eggs could be fair and convenient too. Just an idea....
Meh, the problem isn't the spawners. Spawners a really clean way to gain levels that are needed for mass-enchanting. Mass enchanting is a good way to make money on a decent economy server. Selling enchantments you don't need and tools. The real problem is the fact that these items can be sold in the current market, that shouldn't be a thing. I agree that eggs should be purchasable or at least craftable but still I digress. I understand your concern for the economy but this really wouldn't solve anything.
It's also entirely possible to code the spawner-generated enemies to not drop items and only drop vanilla EXP.