Major Sandbox providers (e.g. Modal) run on non-hyperscaler bare metal not AWS and so don't need to markup on AWS's markup. Thus, prices are comparable or better than AWS.
In that case it's still overpriced because they're charging hyperscaler prices without offering a hyperscaler level service in terms of scalability, reliability, security, trust, etc.
In that case it's still overpriced because they're charging hyperscaler prices without offering a hyperscaler level service in terms of scalability, reliability, security, trust, etc.