I don't think anyone should be seriously considering the cost/benefit of the US annexing Canada - it just serves to legitimize the very real and scary threats made by Trump. His statements amount to a declaration of war and should be treated as such.
Realistically if we accept the idea of Canada joining the US they'd likely join as their current provinces (then states).
This would allow Canadians to have a voice in US elections which currently have large political implications for the nation.
I don't see Quebec joining the US. Current Canadian law grants them protections the US wouldn't be willing to match. They'd likely become independent.
Despite Canadian identity being based on not being Americans there are large cultural similarities (given English structural roots).
If it were to occur Canada would likely get some consideration like the preservation of their healthcare system (a point of national pride) and a period for other states to join in on it.
The US would be more blue politically but I would expect catering to northern provinces given they'd have disproportionately high representation.