takasi said:
The best fix is to limit the potion's shelf life to one day. This should match the prices above and still make the animal valuable.
Still, I can see this lowering the usefulness of the item considerably. The advantage of potions is that they can be taken just about anywhere and used days, weeks, months, even years after they are made.
So how about the item being good for, say, 7 - level * 1d3 days. By this I mean something like the following:
An egg carrying the property of a 1st level spell would be good for 6d3 days. The shopkeeper could guarantee it is good for 6 days, but beyond that there is a chance it is past its shelf life, and there should be a table of potential mishaps should the item be consumed (with greater / worse effects the further beyond its shelf life).
A cup of milk carrying the equivalent of a 6th level spell, on the other hand, would only be good for 1d3 days, guaranteed for only 1 day.
This would allow for some leniency in that the party could travel a bit before the item were rendered useless. After all if such a creature existed, and the shelf life of its products were good for only one day, it would only be useful for the local town (excluding the use of teleport, magitech transport, etc). If it can last a few days, its products can be distributed over a much larger area, making it more useful in the long run.
(7 - level) * 1d3 days
(7 - level) days - guaranteed effect (no chance of ill or no effect)
Actually rolled number of days - no chance of ill or no effect, but beyond guaranteed period this probably unknown, thus adding in an element of risk. Is it really okay to drink the milk past its (magical) shelf life, or will the character grow horns or hoofs if they drink it?
Beyond actual rolled number of days - roll on a table to determine whether no effect or an ill effect occurs - with ill effects perhaps based on the school of magic? Maybe a minor necromantic ill effect could include nausia, while a major divination ill effect could include blindness? Something like that, anyway . . . .
The potion makers would deride the item, based on the fact that it has such a short shelf life and can have ill effects if used past its due date, while the breeders of such animals would point out that their products cost far less and can be mass produced, added to meals, etc. It could be an interesting situation, I think.