EWB,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
Количество монет: <input id="gold" type="text" value="100000">
<br/>
<br/>
Сумма(в рублях):
<input id="result" value="367.5" type="text">
<script>
var input = document.getElementById('gold');
var input2 = document.getElementById('result');
input.oninput = function() {
input2.value = (3.5 * input.value / 1000 + 3.5 * input.value / 1000 / 100 * 5).toFixed(2)
}
input2.oninput = function() {
input.value = (input2.value / (3.5 / 1000 + 3.5 / 1000 / 100 * 5)).toFixed(0);
};
</script>
</body>
</html>