Always go for quality over quantity. 5 good quality backlinks better than 50 random backlinks.
2 strategies I recommend that won't cost you:
1) guest posting, just go in google and search for this:
"Your Niche" + "blog for us" OR "guest post" OR "write for us" OR "become a contributor" OR "guest column" OR "guest author" OR "guest article"
just replace Your Niche.
2) Broken link building, you'll need to install chrome extension called Check My Links then you'll search for relevant pages from websites you want to get backlinks from and you run the plugin to see if there are any broken pages on those articles so you'll contact the websites and ask to replace those with your content.
