The chances are very close to 0. The amount of calculations needed for this is incredibly large. Suppose you already know the set of 12 words, but not their order. The number of possible permutations is 12! (12 factorial), which is 479001600.
So far you're correct, although the possibility of duplicate seed words would reduce the number of permutations.I did not know that duplicates are allowed, never saw such a seed.
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But there are much more words used for seeds. A meager set of 100 gives the number of permutations of about 9E157 (9 with 157 zeroes), and only millions of them correspond to existing wallets.
Wrong! You're talking about 100 different words that you have, but don't know the order for. 12 words out of 100 possibilities gives 100^12=10^24 possibilties. A 1 with 24 zeroes, you're a factor 9E133 off, which is (based on a high estimate) about 20 sexdecillion times more than the number of atoms in the universe. You, sir, added a new level to "you couldn't be more wrong" 
Well, my knowledge of combinatorics is a bit rusty, so I forgot to account for the set size. Still, even if there is a possibility to check 1000 combinations per second (and, AFAIK. key derivation and address check are actually much slower), checking all 1E24 combinations would require the amount of time orders of magnitude larger than the age of the universe.