Rarities?

[Dream]

Retired donation creatures are usually the most valuable. The donation creatures as of now are the Red Panda (2 shards, worth about 20k) the Dark Bone Dragon (3 shards, worth about 30k) and the Molten Bone Dragon (5 shards, worth about 50k). You get shards by donating to Magistream -- $1 = 1 shard. You can buy donation creatures off of other people too, though -- 1 shard = 10k.
And if you get Rose Imps from the quest, you can sell them for a lot of gold or trade them for retired donis. I traded one for a Vanil Shaa and one for a Purple Crystalwing + Ashen Shaa (all 3 of those are retired donation creatures).

[Umalia]

I would like to add a piece of information to acceptable's.

The prices she stated for current donation creatures is the usual prices, however this month's prices are unusually high. There are not as many current donation creatures being offered for sale and they are very popular, causing a lower supply than usual coupled with a higher demand than usual.

[PyroJack]

And if you get Rose Imps from the quest, you can sell them for a lot of gold or trade them for retired donis.

For now. Expect that to stop when the "breed" function is enabled. The prices on them will immediately plummet as cheaper, bred ones become available. They'll probably settle into the same ~30k for QB, 3-5k for 1st-gen, the same as every other Quest creature.