I run an agency and we have shifted from paid services we resell to mostly internal work. Pretty much all of our work is now done in house, apart from press release distributions.
Your question is vague, are you looking to pay for SEO services, or are you looking to do SEO for your own website and pay for services.
If you are wanting to do it yourself, it will take a lot of research and time learning, but you could do it on a tight budget of a couple hundred per month, buy quality guest posts, press release distributions, maybe pay for a once off on page optimisation or strategy plan from an agency or an SEO freelancer. We offer DIY roadmaps and strategy sessions if you are wanting an actionable roadmap.
Our best result is a client we took from a brand new business and website to #1 for 10 keyword in 5 months.
They generate around 700-1000 clicks a month, and receive around 5-10 enquiries each day.
They have sent over $1’000’000 of quotes.
If your niche is not super competitive and your site is already ranking on page 3-5, theres a chance you can move up with just on page optimisation and some citations.