The average monthly spending on SEO (Search Engine Optimization) marketing can vary greatly depending on a range of factors including the industry, the competition, and the specific goals of the startup. Here's a basic breakdown of potential monthly costs:

SEO Tools and Software:

Subscription to SEO tools like SEMrush, Ahrefs, or Moz: $99 - $999 per month.
Content Creation:

Hiring writers for creating SEO-optimized content: $500 - $12000+ per month depending on the volume and quality of content.
Link Building:

Costs associated with building high-quality backlinks: $500 - $9000+ per month.
SEO Consultancy or Agency:

If you're hiring an SEO consultant or agency, the costs can range from $1000 to $15000+ per month depending on the reputation and service package of the agency.
Technical SEO:

Technical SEO improvements and site optimizations: $500 - $20000+ per month, which can include aspects like site speed optimization, mobile optimization, etc.
Local SEO (if applicable):

Enhancing local SEO can cost anywhere from $300 - $11000 per month, involving optimizations for Google My Business, local citations, and reviews management.

Therefore, as an average, a startup might spend anywhere from $2,899 to $23,999+ per month on SEO marketing.

However, it's important to note that these are just rough estimates, and actual costs can vary widely based on the specifics of the startup and its SEO strategy. Moreover, startups with limited budgets might spend considerably less, especially if they handle some aspects of SEO in-house.