<input onChange="myFunction()" type="number" id="myText" value="0">
<button id="btn">Try it</button>
<script>
function myFunction() {
  var x = document.getElementById("myText").value;
  document.getElementById("btn").value = x;
  document.getElementById("btn").innerHTML = x + " Teklif Et";
}
</script>

veya JQuery ile şağıdaki gibi yapabilirsin.

<input type="number" id="myText" value="0">
<button id="btn">Try it</button>
<script>
$( "#myText" ).change(function() {
  var v = $("#myText").prop("value");
    
    $("#btn").text(v+" Teklif et");
    $("#btn").prop("value", v);
});
</script>