I calculated it as 4% and an old post on Silph Road seems to confirm this, but the recent change may have affected the probability. [...] SGPokeMap refuses to show [...]
The patch affected the probability a lot. Until now, cheaters simply looked on a map or twitter and got their wonders with 100% probability, now they have to pray to the rng-gods too - just like legit players.
4% for wild pkm and 67% for hatches seems to be the propability to get a wonder.