Transfering values from a response to a property

Use below script assertion for the same request.

//Define the following ; edit if required.
def expectedProp2Value = 11
def expectedKeyName = 'prop2'
def keyNameToSum = 'value1'

//Pass the response to parseText method
def pxml = new XmlSlurper().parseText(context.response)

//Find all matching element e; 
def alles = pxml.'**'.findAll{it.name() ==  expectedKeyName && it == expectedProp2Value }*.parent()*.parent()
//Sum the values
def result = alles.collect{ it."$keyNameToSum".text() as float }.sum()
log.info "Sum of the values : $result"


//If you want to use the above sum in the next request; use below
context.testCase.setPropertyValue('SUM', result.toString())

You can try the same online demo (till summation part, rest is specific to soapui) 

http://ideone.com/Rk6gdz

In the next request, use below property expansion where the above SUM is needed. For example:

<sum>${#TestCase#SUM}</sum>