Google Analytics & Google Tag Manager Assistance Required

Description

Hi
I have recently taken over a google ads account which is an ecommerce site and have noticed the dynamic sales values being passed through from google tag manager variables into google analytics and google ads are not matching up. They seem to be 20 - 40% out of the true sales value. There are a few possible reasons for this. It could be down to the bolt on products not being factored in, the profit variable not being set properly. I only have a basic understanding of analytics and google tag manager and need someone to go through this with me and help identify the issue.
Need someone of a good understanding of google analytics and google tag manager and who understands how to setup dynamic sales values in google tag manager.