User blog comment:BonesMcGee/Brainstorm: Potential savings in using the hurry hatch trick/@comment-5039776-20120813200117

Just stumbled across this very interesting blog.

As at today (after the release of the Mesohippus), the actual probability of getting a rare is 88/2088 or 4.21% (see the dino den page). Of these, 51/2088, or 2.44% are non-12 hurry hatch, the rest of the rares (1.77%) being "12s".

Put simpler, you have a 1 in 41 chance of getting a non-12 rare when you buy a random dino for 250 coins. If you don't reject the "12s", your chances increase to 1 in 24.

Of course, if you get a 12 (which will happen 97.56% of the time; i.e. 39 times out of 40), the chance it will be a rare will be just 1 in 55 (37/2037).

Seems the chances are pretty balanced in terms of net cost versus likelihood.