I've read it somewhere, but can't seem to find it again. My question is: In each case what is guaranteed? Is it something like one very very rare is guaranteed and like three or four other rares ranging from VR to R? What else?

WOTC does not guarantee that a case will have 4VR/8R. They say a booster will have X minis.

Generally, they do have the amount of VR's expected. I've had a case with an extra (and one short).

Someone correct me if I'm wrong. There are no very, very rares. It just seems like that because some pieces, Boba Fett characters for example, are more coveted "chase pieces" and harder to find on the singles market because they are unlikely to be let go when pulled.

Thanks guys! The reason I asked was because I recently ordered a KotOR case and will be crossing my fingers for Darth Malak and Mandalore!

while theoretically that sounds correct; personally i think in reality theres a few 'more rare' very/rare pieces. Heres why i think so:
1) the newer the booster; the easier it is to obtain a booster of that particular set and thus have a higher chance of pulling a certain figure. Ex: when GAW was realeased; there were more people buying those booster over JA and thus had a higher chance pulling X figure in there rather than X figure in JA.

2) some boosters were 'reprinted'. Im not sure myself what this means; but ive heard that some boosters (ROTS; all 40-piece sets; LOTF) had more boosters realeased over others (Ex: COTF) so in theory theres less COTF bosters out there than ROTS boosters overall; making all figures in COTF 'rarer' than those of ROTS.

3) and then; of course theres the playability and supply/demand thing. Obviously it will be harder to attain a cad bane than 3po ED. Though if you pulled X amount of boosters from both GAW and IE you would have the same chance of pulling either fig. so; attainability and probability of pulling are two different ratios.

sorry; i just looked at all i wrote and realised id gone on a nar-rant. oops :P

Though Tamtel and Boba BH are from the same set, so the comparative ratio should be the same.

Anyhoo, I'd say another major factor in the perception of rarity of an individual figure is the individual booster purchases. As when you buy a pack off of a shelf, its not all from one case, but from a randomized slurry of multiple cases, which can easily lead to getting duplicates of some pieces and never encountering others.