A note to the reader: I randomly chose most of these numbers. Feel free to post with suggestions.
Contributors to pricing eggs:
Base Value - you should use 1,000 tu as a base value because you can let a creatu go for 1,000 tu. In-season seasonal eggs should start at 800,000 tu, out-of-season seasonal eggs should start at a little more, 1,000,000 tu, and Uldavi/CS eggs should start at a little more - 1,800,000 tu.
Age - older eggs are worth less. Halve the value of an egg for every stage it goes down.
Rarity - rarer eggs are worth more. Add 100 times the rarity subtracted from 100.
Contributors to pricing creatu:
Base Value - base value is the same for eggs and creatu.
Age - time is money. Add 100 times the age of the creatu.
Color - not all colors are equal. Add 100 times the color rarity subtracted from 10. Color rarity can be found in the Hatchery.
Rarity - rarity is the same for eggs and creatu.
Effects - effects like Derp, Stardust, and Frost Light are valuable. add 500,000 tu per effect on the creature.
Custom Art - if your creatu has custom art it is REALLY valuable. Add 5 billion tu to the worth of your creatu if it has custom art.
Mutant: mutants are rather difficult to obtain. Add 50,000 to the worth of your creatu if it is mutant.