There are a few AWS security services which are free/priced reasonably.

Some free services:

1. AWS Org (Disable services and enforce guardrails)

2. VPC (Create private networks)

3. IAM (User access and IAM policy analyzer to help with least priv)

4. IAM Access Analyzer (Alert on resources with cross account & public access)

5. SSM Inventory & Patch manager (Basic check if all VMs have security updates installed)

Reasonably priced IMO:

1. AWS WAF with free managed rules (when rightly configured you get lesser FP and high ROI)