var massiv = [ { "key": 0, "duration": 4, "price": 1000000 },{ "key": 1, "duration": 6, "price": 1250000 },{ "key": 2, "duration": 12, "price": 1500000 }, ];

var elementString = "";
for (var i in massiv) {
  elementString += ''+massiv[i].price+'';
}    
elementString += "";

console.log(elementString)