I suspect that $1500 a month might not be ballpark.

As said - cold callers are looking to their pockets, not yours. Generally.

The ideal scenario is for the SEO optimiser to be paid a commission on sales over your baseline average. But I doubt too many cold callers would agree to that. And for $1500 per month I would be wanting them to generate a minimum of $25,000 per month. Upfront.