This is an open thread, for all of you who like to buy phyiscal forms of manga, either due to personal preference or you are a collector (I do it for both reasons). Feel free to put your own stores down here, this is just from my findings. Not all prices will be the same across all manga, but this is just a base price.
I don't know about in the US or anywhere else, but book stores that include anime and even pop culture stores never seem to have extra stock. Which usually means for every anime, the first one is gone, the middle ones are gone and you have the last ones in abundance.
This is why I took to online shopping and I went around all the places that sell anime to find the greatest deals.
My final judgement is the best place to buy manga online is http://www.bookdepository.com/
Manga is prices at $10 USD each, with free, fast shipping worldwide.
Disclaimer: This is if you would like to buy the volumes new, if you are wanting to buy second hand manga, then I would suggest you go on Amazon basically no matter where you are because they are about $1 each, so even if posting is a few dollars extra (for international), it probably won't equate to more than the $10 USD that Book Depository offers - but I would check it up first.
For those in America, your Amazon store has great prices on Manga at about $7.64 USD per volume. That is not including postage so take that into account - but if you have Amazon Prime then this may be eligible for you to get free postage and therefore this will be your better option.
For those in the UK, Amazon is not your best option as your pricing is about 9 pounds, which equates to about $11.50 USD.
For us in Australia, Manga is usually $14 USD at it's lowest in physical stores, and honestly even on eBay from Australian retailers. eBay is not really the best choice, even for second hand Manga, it is a lot cheaper on Amazon.
If you are en eBook reader with either a Kindle or Comixology account then the prices are about $7.34 USD per Volume, that obviously comes with no shipping costs. Also, Comixology has a subscription service which includes an abundance of comics (not DC or Marvel) and manga (includes Attack on Titan) for about $6 USD a month.
I hope this helped out 🙂 I am always looking for a good bargain and every dollar counts if you buy shit loads of manga like I do.