The SEO service cost veries based on:
1. Agencies or consulting firm geographical located.
2. Quality of service.
3. Man-hours invested.
4. Complexity of project.
For more information refer below links:
https://www.rankpay.com/seo-cost/
https://searchenginewatch.com/sew/how-to/2267471/how-much-should-you-spend-on-seo-services