I don't know of any way to answer this question. The only question I know of a way to answer is, how much will animal food cost if it has to compete on a level playing field with other protein sources? We can answer that question by simply having a free market in food. If enough people's preference is to not eat food that involves animal suffering to produce, then the free market will result in that kind of food not being produced any more.
(Btw, it's not necessarily true that animals raised for food will suffer. It's perfectly possible to raise them humanely and kill them when the time comes in a way that causes no suffering. In a free market that might not even cost more than factory farming of animals does today, since animals raised humanely are generally healthier and require much less artificial intervention such as antibiotics, which are routinely fed to factory farmed animals because of the artificial environment they live in.)
For market forces to work, we need to attribute some monetary value to the quality of life of animals.
Eggs from chicken that have more space and can go outside suffer less but are more expensive to produce.
Ignorance or questionable ethics should not be rewarded by monetary gain. That is why relying on consumers to "vote with their wallet" is a fundamentally broken design.
External costs should always be included in the price of a product.
> For market forces to work, we need to attribute some monetary value to the quality of life of animals.
In a free market, there is no "we". Each individual decides what to buy at what price based on whatever criteria they like, and each seller decides what to sell at what price based on whatever criteria they like. They everyone continually adjusts their choices based on what results they observe. There is no central authority that decides what anything is "worth".
> Eggs from chicken that have more space and can go outside suffer less but are more expensive to produce.
Generally, yes, which means you, as a buyer, have to be willing to pay more in order to provide a positive incentive to producers that do this. Which is exactly what my wife and I do when buying eggs (and many other things); we choose to pay more to reward producers that do things in a humane way and thus incur higher costs.
> Ignorance or questionable ethics should not be rewarded by monetary gain.
You are perfectly free to not buy from producers that don't do things the way you think they should be done.
> That is why relying on consumers to "vote with their wallet" is a fundamentally broken design.
If other consumers disagree with you, then the way for you to "fix" that "problem" is to convince them to change their buying decisions. Trying to get a central authority to dictate who can buy and sell what does not work; no central authority can aggregate all of the necessary information. It's mathematically impossible.
> External costs should always be included in the price of a product.
In general, the only viable way to do this is to not have externalities: to allow property rights to be traded so that all externalities get internalized, i.e., they are explicit costs or explicit benefits to one of the parties to the transaction. That ensures that they get properly taken into account.
(Btw, it's not necessarily true that animals raised for food will suffer. It's perfectly possible to raise them humanely and kill them when the time comes in a way that causes no suffering. In a free market that might not even cost more than factory farming of animals does today, since animals raised humanely are generally healthier and require much less artificial intervention such as antibiotics, which are routinely fed to factory farmed animals because of the artificial environment they live in.)