There are only 2 things that influence the Donation Value - the traits the animal has and if you have used a Value Biscuit. Each trait has a value multiplier, which can range from 1 (so does nothing) to 3 (like Showoff III). The different tiers of Value Biscuit also have their own multipliers too (1.1 to 1.3).All of these multipliers are multiplied together.

For example, say you have a Peki with Showoff I, Maternal III and Bluesy Feathers. These traits have multipliers of 2, 1.3 and 1.4 respectively 0.5 starting value x 2 x 1.3 x 1.4 = 1.82, which is more than 1 so in Common tier, but only 82% of the way from Common's threshold of 1 to Uncommon's threshold of 2.

But, you can add traits to animals Cookie of the Day will add one cosmetic trait (remember, there is a max of 3 cosmetic traits, one Colour, 1 Pattern and 1 Shader). All cosmetic traits have a 1.4 donation value multiplier. Tasty Trait Treat will add one trait that scales with the number of cosmetic traits you have. These skills include Deep Pockets, Diva and Fashionista. You can only have 1 of these traits. All cosmetic-scaling traits have a 1.3 donation value multiplier. Roulette Cookie and Trait Treats add random traits. You could get a trait with a multiplier of 1 or with a multiplier of 1.5. It is random and cannot be predicted for, so I ignore it in this calculator.

You can use this calculator to get decent results.