If you've got a tablet I'd recommend trying purchasing your comics digitally from Amazon or Comixology (digital comics service owned by Amazon). The digital offerings are often cheaper than the same content in print, and that can stretch your dollars.
Cecil made great recommendations. For Astonishing X-Men, you'd get volumes 1-4 written by Whedon to get the complete arc. On Amazon, the print cost for the four volumes is about $50 USD, and the third volume is only available used. For the digital editions of the same 4 volumes, you can get them for $13 USD.
https://www.amazon.com/Astonishing-X.../dp/0785115315
Long Halloween is a great self contained Batman comic. $17 USD on Amazon for the print version, $5 USD for the digital.
https://www.amazon.com/Batman-Long-H.../dp/B0064W639K
I don't know Batman comics but I would trust Cecil's recommendation of Dark Victory based on his other guidance.
If you're looking for content that is a little darker but that doesn't involve superheroes, consider
Saga (sci-fi), or
Southern Bastards (kind of crime noir w/a Friday Night Lights twist), or
Lazarus (near future dystopian sci-fi), or
Deadly Class (80s coming-of-age but w/assasins!). If you do buy from Amazon, double check the prices between the US and Canada sites as there can be significant price differences. Also check between Amazon.com and Comixology, as even though Amazon runs both sites there can again be significant price differences.
EDIT:Whoops, missed the comment about non-Marvel and non-DC publishers. Sorry for the hard sell!
If you'd rather spend local, all books I've discussed could be found at great local shops such as
Phoenix Comics NW on 16 Ave by SAIT, or
Alpha Comics in the SE.