Why are my Google shopping campaign costs high with low conversions?

I would say your quality score is low due to your website needing to look better. Google will reward you for a great looking store and your CPC will be lower. What sort of numbers are you getting at the moment and how many visitors to your store. If people are coming to your your store and leaving quickly with no sales you know your store isn’t good enough. Shopping ads can take upto 3 months to fully optimise and you need to give people a reason to buy from you rather than Amazon.