Check to see how many outputs total for the tx using .length and then do a for loop and run through every output for each transaction. Use an if statement to check if that particular output has your desired address in the 'addr' field. If it does match then print that value to the screen.

I made a fiddle that I think should work, I didn't test it though... http://jsfiddle.net/nmgus1fa/

var btcs = new WebSocket('wss://ws.blockchain.info/inv');

btcs.onopen = function()
{
btcs.send( JSON.stringify( {"op":"addr_sub", "addr":"15wsV4TYvGPrJnhoyCed6kWcNUuSo5ykRi"} ) );
};

btcs.onmessage = function(onmsg)
{
  var response = JSON.parse(onmsg.data);
  var getOuts = response.x.out;
  var countOuts = getOuts.length; 
  for(i = 0; i < countOuts; i++)
  {
    //check every output to see if it matches specified address
    var outAdd = response.x.out[i].addr;
    var specAdd = "15wsV4TYvGPrJnhoyCed6kWcNUuSo5ykRi";
       if (outAdd == specAdd )
       {
       var amount = response.x.out[i].value;
       var calAmount = amount / 100000000;
       $('#messages').prepend("<p>" + calAmount + "</p>");
       }
  } 
}