How to get value from “dynamic row field” column to another field in system.xml. How to compare this with Order Subtotal after proceed to checkout

How can to get value of “price” column in “custom Alert Message” Field.
here is system.xml code `

<field id="ranges" translate="label" sortOrder="5" showInDefault="1" showInWebsite="1" showInStore="1">
                    <label>Set Tax</label>
                    <frontend_model>MyCompanyOrderBlockAdminhtmlFormFieldRanges</frontend_model>
                    <backend_model>MagentoConfigModelConfigBackendSerializedArraySerialized</backend_model>
                </field>
                 <field id="alert_message" translate="label" type="textarea" sortOrder="40" showInDefault="1" showInWebsite="1" showInStore="1">
                    <label>Custom Alert Message</label>
                    <comment>Enter custom alert message .</comment>
                    
                </field>`

.How can i get “price” field value from system.xml and “Customer group”, when customer clicks “Proceed to checkout” and then compare the value with order subtotal.And On the basis of it how to sieze “checkout process” and redirect customer to “checkout/cart/” so that he can update order to match the validation.