A good post on stackoverflow here describes how this works. Look at the solution, it's surprisingly simple using the flow.interview variable

http://stackoverflow.com/questions/10032799/how-do-i-get-a-variable-back-from-a-flow

Unfortunately the Apex documentation hasn't kept pace re. how to use flow.interview

The essencial part is, creat your flow.interview.My_Flow_Name variable in the constructor. Create a finish location method, and in that method pick up the variable directly from your flow.interview.My_Flow_Name,

ex.

VF Page uses a typical tag like this

<flow:interviewname="My_Flow_Name"interview="{!myFlow}"finishLocation="{!finishLocation}"/>

And the controller for the page

/* VF controller class */
public with sharing class myFlow_VFCx 
{
flow.interview.My_Flow_Name myFlow {get;set;}
 
public myFlow_VFCx(ApexPages.StandardController stdController)
{
     //Your logic in here
 
     //Pass value(s) TO the flow like this:
     Map<String, Object> valueMap = new Map<String, Object>();
     valueMap.put('Flow_variable_unique_name',someValue);
     myFlow = new flow.interview.My_Flow_Name(valueMap);
}
public getFinishLocation()
{
     //GET Values from a flow like this
     return new PageReference('?'+myFlow.My_New_Contact_Id); //Where My_New_Contact_Id was a variable's unique name in the flow
}
}

Note the common confusion in using flow in apex is that the flow name is like a variable, it isn't - flows are stored as Meta data, like your custom objects, so when you are referring to My_Flow_Name you can think of it as if you are referring to an object.