Ok, so as I suspected they rejected us based on the grounds that they were not satisfied on the grounds that I didn't show satisfactory intent on moving back home. I gave them a solid plan, conversations with my parents telling them that we are excited to come back, house search and quotations, job search back in Montreal, etc.
This actually sounds pretty weak.
But I will definitely appeal this. Does anybody know how long and what costs are involved with appealing?
I strongly suggest you look into how the appeal process works and how long cases take before making any decisions. (Although since you only have 30 days (?) to decide to appeal, nothing wrong with filing the appeal while you decide, I believe you can withdraw it at any time).
This thread (long) has lots of appeal cases:
https://www.canadavisa.com/canada-immigration-discussion-board/threads/all-spouse-appeal-cases-come-here-and-join-us-plz.87619/page-1132
You'll note very few of them go quickly.
And refer to the first posts in this thread where you asked: personal opinion only, but I think you are better served and it will go more quickly for you to move to Canada, forget about the appeal, and file for PR from within Canada.
The worst decision anyone can make, usually, is to decide to 'appeal' or 'go to court' because they think they're "right" and they're angry. It's a pure rational decision: which will get you the better result, more quickly, cheaper. (Most who are appealing are doing so because they have no other choice). The reason you're being told to just apply again (personal opinion) is that all indications are that appealing will take more time than starting over (in your case).
And a quick point about the appeal: even if you 'win' (which might take 6-12 months, and often longer), all that happens is they process your file starting all over again; you've 'won' that they won't turn it down for the same/previous reason, but you're still looking at ... yet another ~12 months or more. (Note I'm not an expert on this, but from what I've been able to understand about the process).
But up to you, it's your right of course.