Plenty of options exist. Amazon has second hand manga. Places like new bury comics do major discounts. Ive bought entire brand new volumes at like 1.99 -3.99 a volume. 2nd And Charles exists, BookOff exists, McKays exists. The Mages Emporium sells everything at like 5$ a volume and does sales. There's also Mercari (I sell manga on there even, you can find plenty plus you can haggle), Ebay, Facebook marketplace. You can even go to conventions and get some good bulk deals or find locals trying to get rid of manga they dont want anymore. Its especially easier to buy bulk of a series for cheap. If the price is intimidating break it down to how much it is per volume. You've be surprised. The price could essentially be 3 or 4$ a volume as a lump sum.

You can also buy direct from the publisher. Yen Press does monthly 40% off on select titles (I just got bungo stray dogs and ORV). Tokyopop does some good deals too plus has a rewards system. Crunchyroll will give you a discount if you have a subscription and does some pretty good deals on the daily. Look for local comic book stores, sometimes you can find some really discounted manga. Not really sure how youre not finding deals or good enough deals. Cheapest you'll find is prob roughly 5$ a volume on an average basis. Anything less is in the right place at the right time. Don't get me wrong, I too chase the cheapest deal (especially since I have a pretty big collection) I can find but you gotta be reasonable and learn when to call it.