You mean the cost of the back links corresponds to the authority of the site which is providing the backlink. Right?
in theory, yes. But, like with most things in life and especially in SEO 1+1 does NOT equal 2 all the time.
That's me saying that there are scammers who overinflate their authority (the DA / DR metrics of their sites) and sell you backlinks from those artificially inflated sites for more money than they're worth. You really need to make an effort to learn how SEO works. I can just explain it to you in a nutshell, but the finer details and the various nuances you'll have to test, read about and figure out on your own because very few people know EXACTLY how google works, and those few that have this insider knowledge are probably somewhere on an exotic beach, drinking their pina colladas at the moment, they're not on this forum giving the secrets away for free to strangers like you and me
But anyway.... in theory, SEO is simple: you need quality content (which, by the way, you don't have to pay $300 for lol) to keep the traffic (the people) that google sends you interested in your content, thus in your site, and thus in your recommendations (products / services that make you money), and then you need backlinks to keep your competition away from the delicious #1 spot in google that everyone fights for.
So, in a nutshell, SEO = great content + great backlinks. These are the 2 most important factors that will usually decide if your site ranks, for which keywords, and where.
But, like I said, this is the general picture. In reality, SEO can be broken down into more than these 2 pieces, and those pieces include: anchor texts, domain age, number of words on your site, engagement with your visitors, site design and structure, site-specific technicalities (404 pages, broken links, etc.) and many more.
And then there are the sandbox and the exclusive niches like medical / banking / lawyers (well, the financial niche in general) which not every average Joe can break into without deep pockets and without some advanced SEO knowledge, there are also things like google ads, spamdexing and many more, and each one of these have their important role in SEO.... You need to do a lot of reading, man, and you need to test things yourself to see what works and what SEO is all about in the different scenarios (different niches, different domains: expired vs new, different keywords, etc). And then... when you've figured it all out google rolls another important update and we all start from scratch: asking around, testing, trying to figure out what they have targeted this time, how to fix it, and so on. SEO is a long term game and an ever-changing one, too, so be prepared for it and understand how it works or else you're just another cash cow for some smart / experienced SEOers (who even told you that a piece of content costs $300?? that's absurd...)