If you’ve used a different meta key than the original plugin, your examples “acf_cost_price” instead of “cost_price” then this should have no affect on the original plugin field.
The only thing that can cause this is using the same meta key, the ACF field name. When using the same meta key then only editing only one of the fields will work and this will be the field that is saved last this value will overwrite whatever value is saved first.
