There is no set answer to what a reasonable SEO budget for new startups is, as it can vary greatly depending on several factors such as the industry, target market, competition, and overall business goals. However, it is generally recommended that small businesses should allocate at least 8-10% of their total revenue towards marketing, with a portion of that being dedicated to SEO efforts.

As for the average SEO budget per month for startups in the US, it can also vary greatly depending on the factors mentioned above. According to a survey by Ahrefs, most small businesses spend between $500 to $5,000 per month on SEO services. However, this can be significantly lower or higher depending on the specific needs and goals of the startup.

Ultimately, it's important for startups to assess their own budget and goals, and work with an experienced SEO professional or agency to develop a customized plan that fits their needs and budget.