Compliance certification, often referred to simply as "certification," is a process by which a product, process, or service is evaluated to ensure that it meets specific standards, regulations, or guidelines set forth by a regulatory body, industry organization, or other governing entity.
For example, in the context of environmental certifications like GREENGUARD, compliance certification involves testing and verifying that a product meets certain standards for indoor air quality. Once a product has been certified as compliant, it can be labeled or marketed as such, providing assurance to consumers that it meets certain health, safety, or environmental criteria.

A standard is a set of guidelines, specifications, or requirements established by a recognized authority, such as a government agency or a professional organization, to ensure consistency, quality, and safety in a particular area. Standards can cover a wide range of topics, including products, processes, services, and systems.

ISO 9001:2015 is a globally recognized standard for quality management systems (QMS). It is part of the ISO 9000 family of standards, which provides guidelines and requirements for organizations to establish, implement, maintain, and continually improve a quality management system.

When choosing a certification body, you should:
Evaluate several certification bodies.
Check if the certification body uses the relevant standard
Check if it is accredited. Accreditation provides independent confirmation of competence. However, accreditation is not compulsory, and non-accreditation does not necessarily mean the certification body is not reputable.

There are various factor on prices:
Depends on standard, ISO 22000 certificate price is different than ISO 9001.
Depends on Certification body, all certification body has different price list.
Depends on number of employes, and audit man days.