Hi Ryan,

You can create a Custom Object like 'Child Opportunity' which will have a Lookup Relationship to Opportunity. Add fields like Item, Price and Opportunity__c (Lookup field) to this object.

You can then create a field like 'Sum of Child Opportunities' on the Parent Object, Opportunity. Any Price values added/updated/deleted in the Child Opportunity record will reflect in the related Opportunities' Sum of Child Opportunities field. 

(This functionality is exactly same as a Roll up Summary field which can be achieved by creating a Master-Detail relationship between Opportunity and Child Opportunity. However, I guess you cannot add another Master-Detail relationship on Opportunity (apart from Opportunity Product).
https://success.salesforce.com/answers?id=906300000019MPAAA2

In that case, you will have to achieve this Roll-up Summary functionality through some other means, either by using a Trigger or a Process Builder.

I tried with Trigger and it works fine.
 

trigger ChildOpportunityTrigger on Child_Opportunity__c (after insert, after update, after delete) {
    
    ChildOpportunityHandler handler = new ChildOpportunityHandler();
    
    if(Trigger.isAfter){
        
        if(Trigger.isInsert){
            handler.addInSumOfChildOppField(Trigger.New);
        }
        
        if(Trigger.isUpdate){
            handler.updateSumOfChildOppField(Trigger.Old, Trigger.New);
        }
        
        if(Trigger.isDelete){
            handler.deleteFromSumOfChildOppField(Trigger.Old);
        }
    }
}

 

public class ChildOpportunityHandler {
    
    public void addInSumOfChildOppField(List<Child_Opportunity__c> newChildOpportunities){
        
        for(Integer i = 0; i<Trigger.size; i++){
            
            if(newChildOpportunities.get(i).Opportunity__c != null){
                
                Decimal sumOfChildOpportunities;
                Opportunity opp = new Opportunity();
                opp.Id = newChildOpportunities.get(i).Opportunity__c;
               
                Opportunity oppor = [SELECT Sum_of_Child_Opportunities__c FROM Opportunity WHERE ID =: opp.Id]; 
                
                if(oppor.Sum_of_Child_Opportunities__c != null){
                   sumOfChildOpportunities = oppor.Sum_of_Child_Opportunities__c; 
                }
                else{
                    sumOfChildOpportunities = 0.00;
                }
                opp.Sum_of_Child_Opportunities__c = sumOfChildOpportunities + newChildOpportunities.get(i).Price__c;
                update opp;
                
            }
        }
    }
    
    public void updateSumOfChildOppField(List<Child_Opportunity__c> oldChildOpportunities, List<Child_Opportunity__c> newChildOpportunities){
        
        for(Integer i = 0; i<Trigger.size; i++){
            
            if(oldChildOpportunities.get(i).Price__c != newChildOpportunities.get(i).Price__c && newChildOpportunities.get(i).Opportunity__c != null){
                
                Opportunity opp = new Opportunity();
                opp.Id = newChildOpportunities.get(i).Opportunity__c;
                
                Opportunity oppor = [SELECT Sum_of_Child_Opportunities__c FROM Opportunity WHERE ID =: opp.Id]; 
                
                opp.Sum_of_Child_Opportunities__c = oppor.Sum_of_Child_Opportunities__c - oldChildOpportunities.get(i).Price__c;
                opp.Sum_of_Child_Opportunities__c += newChildOpportunities.get(i).Price__c;
                update opp;
                
            }
        }
    }
    
    public void deleteFromSumOfChildOppField(List<Child_Opportunity__c> oldChildOpportunities){
        
        for(Integer i = 0; i<Trigger.size; i++){
            
            if(oldChildOpportunities.get(i).Opportunity__c != null){
                
                Opportunity opp = new Opportunity();
                opp.Id = oldChildOpportunities.get(i).Opportunity__c;
                
                Opportunity oppor = [SELECT Sum_of_Child_Opportunities__c FROM Opportunity WHERE ID =: opp.Id];
                
                opp.Sum_of_Child_Opportunities__c = oppor.Sum_of_Child_Opportunities__c - oldChildOpportunities.get(i).Price__c;
                update opp;
                
            }
        }
    }
}

Try adding records of Burger, French Fries, Coke in the Child Opportunities object along with their prices and associate it to an Opportunity. Check whether Opportunity's Sum of Child Opportunites field value is getting changed on every Insert/Update/Delete or not.

Let me know if there are any issues.