[Sigizmund2012]

Ramzes94,

<input id="price" type="text" placeholder="Цена">
  <label>Добавить тысячу<input type="checkbox" id="plusthousand"></label>
  <label>Поставить тысячу<input type="checkbox" id="setthousand"></label>

  <script>
var price = document.getElementById('price');
var plus = document.getElementById('plusthousand');
var set = document.getElementById('setthousand');

plus.onchange = function(e){
  var pricevalue = parseInt(price.value);
  if (this.checked === true && !isNaN(pricevalue) ) {
    price.value = pricevalue + 1000;
  }
};

set.onchange = function(e){
  if (this.checked === true) {
    price.value = 1000;
  }
};

  </script>