Anyone who has opened a decent amount of MTG boosters will notice that they get much more of some tokens than of others. I understand that this comes from varying needs for them, because far fewer people are going to need a planeswalker emblem than need a spirit token, but are there different rarities for tokens like there are for regular cards? Or does each token in a set have its own unique frequency of occurring?
I couldn't find an official answer, but I can make an educated guess based on how cards are produced in general.
Cards are printed in big sheets and then cut out and randomized. These sheets are grouped by rarity. You print sheets based on the desired ratio of rarities in the final product, e.g. 1 rare sheet per 10 common sheets if the two sheets are of equal size.
Sometimes, more than one copy of a cards is on each sheet. For instance, early sets, particularly the small sets which tried to simulate three rarity levels with only two print sheets, had multiple copies of some cards -- this is why Camel is listed as "Common 5" in some collector guides, or why Knights of Thorn is an "Uncommon 1" (functionally a rare for The Dark). For a more modern example, consider mythic rares: mythics don't get their own sheet; instead the rares sheet has two copies of every vanilla rare and one of each mythic.
This is undoubtably how the ads and tokens sheet is arranged as well: all of the ad and token cards are printed on one sheet, but there are multiple copies of the "common" tokens and few copies of the "rare" ones. For example, a whole sheet of around a hundred M13 ads and tokens might contain twenty goblins and only one Liliana emblem. The exact mix of cards is likely to vary from set to set, but you can get a good idea of the rarity of "rare" tokens if you know the overall sheet size.
I've seen it suggested that token rarity is proportional to the rarity of the least rare cards that produce those tokens. So you would find many more of a token that is produced by a common card, than a token which is only generated by a Mythic.
Seems like the actual policy hasn't been made public by Wizards, but it seems like this would be a sensible route to take, and it's probably a good rule of thumb to work by, in terms of whether a given token is likely to be easy or hard to come by. No one seems to have provided compelling evidence that it doesn't work something like this; though of course there will always be people who have opened boxes with anomalous token distributions in. Isn't that just how opening boxes of Magic cards works?