1) It's worth
2) There are many seo agencies today to fit any budget. But be careful, saving money by choosing the cheapest agency may cost you way more in the future (may get punished by Google for black-hat SEO for example)
3) Basically, SEO is divided into Onpage and Offpage (linkbuilding). You can do basic stuff yourself: Titles, Descriptions, Headings. It won't be perfect but it's free =) Also, I'd suggest checking backlinko.com if you have time and will to dive a bit deeper in SEO.
The truth about linkbuilding is that it's expensive. It doesn't matter if you build links yourself or pay others to build them, white-hat seo costs money as it requires a lot of time. You can't just buy 1000 links for 10 cents anymore.
Answering your question "how to get the site "known" -> write the best content you can -> do onsite SEO (URLs, Titles, etc) -> look for communities, blogs interested in your topic and contribute to them.
Sure, if you can invest at least $500 per month you can find a freelancer to do the job. But don't trust claims like "$100 for top 3 in Google".