How do I reference a second level custom object on $sorter token?

Hi

We are using the formula below successfully in a token which brings the last product for a Membership. 

#set( $MembershipMostRecent = $sorter.sort($Membership__cList,"CreatedDate:desc") ) 
#set( $LatestMembershiDdetails = $MembershipMostRecen[0] ) 
The product of the latest membership is ${LatestMembershiDdetails.Product_Name__c}

Now we would like to bring the latest price which only sits on the Membership>>renewal. 

How do you reference Membership renewal list on the formula above?@s

Marketo Price Field

${Membership__cList.get(0).Membership_Renewal__cList.get(0).Sales_Price__c}

thanks for your help, 

Maria Riley