In my experience comparing with other people in my guild, Id say that the droprate is likely 0,2-0,5 % More likely 0,4-0,5 %
That is on average a drop in 200-250 polish/motivations. - A bit less for plunder.
The first BP was seemingly at an enhanced chance (on each server).
First one dropped for me as early as the 3rd attempt, usually before the 10th and at the latest on the 24th attempt. (7 servers).
This matches the experience of the guild mates Ive heard from.
Its hard to get a completely accurate picture, of course as especially small members/neighbours/guildmates will tend to have a bigger chance to get a polish or even motivation on stone age stuff, which seemingly doesnt have a Great Building and so no chance of a blueprint.
One question on that note, though. - What is a "Well of Welcome". Cant find it anywhere. - Im guessing it was an award for some sort of holiday quest series in the beginning of the game or even a limited introductory offer? All I know is that it can be polished, so I dont think it can be a great building. Not quite sure, though.