Well, some libraries have manga collections that you can borrow for free. That's a cost-effective option. Also, keep an eye on sales and discounts on major online bookstores during special occasions.
One option could be certain file-sharing websites, but be careful as not all of them are legal or safe. Another possibility is to search on dedicated manga download platforms, but make sure they are authorized to distribute the content.
To get free manga, you could look on some community forums where users sometimes share links. But be careful as not all of them might be legal or of good quality.