What if Trudeau decided to build a wall between Canada and the US. Do you think he could get trump to pay for it?

No way - Trump would have the wall financed by another and have them pay him for the use of his name.

 Maybe he could and it wouldn't be the first time Canada played hard ball with the US  and won. Back in the 80's or 90's Canada wanted the US to sign a off shore fishing agreement but they refused. So Canada said - well if you don't sign the papers we might not re new your lease for the Nanoose Bay Maritain Test Range - the next day the US signed the papers. There is only 10 years left on the new lease so maybe Trudeau can remind Trump about that at the Northern Border wall meeting,

he can try

i never wanted Trudeau as a Prime minister, but i dont think he have that in mind so its nice. anyways there been thousand and thousands of migrants coming from the US border to canada to get citizenship.. after trump was elected...

"I think he'd wait until Trump finished the Trump Wall ... the way it wouldn't cost Trudeau anything."

I don't think Americans are flooding the border.  It would be great if all those who promised to leave would leave.  

Estimated about 2000 Americans moved to Canada because of Trump so maybe Trump would be more likely to pay Trudeau NOT to build a wall.