1/333 chance and so is every villager. Actually 1/325 if you've 8 villagers. The only thing that would make it easier to get a villager is if he/she is a high tier because more people would trade them which Rosie is.
Named my town Rosie and Rosie was my starter. She's been stuck in my perfect fruit town since Sept 2013.
Since all my player are in tents and don't speak to them the villagers don't leave.