SEO Costs can be broken out into two categories, things you can do now and get closure and things that go on for some period of time longer.
The first category is fairly cut and dry: identify what the problem is, work on fixing it, then move to the next problem. Simple as that. Like a puzzle, you put a piece down and search for the next piece.
The second category is a bit more complex. Here's where a significant chunk of digital marketing takes place. This is the ongoing process that never really ends. There are always more keywords that need to be optimized, there is always additional content that needs to be published, there's always additional socializing that needs to be done, links that need be found and data analysis is a never ending task.
Don't short-circuit your strategy by putting all your eggs into the short-term cost. You need a long-term strategy that will reap rewards for perpetuity. Look at it as an office space which requires ongoing expenses and maintenance.
Think about these costs as some percentage of every dollar you earn, as if you were putting it toward rent. This cost requires a line in the budget. Costs can always be reduced on one category and increased for the other, but a balance needs to be struck and revisited regularly.