That's a difference between a 5000 per month SEO work and 2,00,000 per month SEO work.
You should seek for excellent people who can give you quality work - just look at their work performance.
1. Get in touch with SEO experts before hiring an agency (paid consultancy)
2. Define clearly what exactly you need
3. Make measurable matrics
4. Once finalize your agency, Give them time & money