[Erolast]

<script>
function increase_balance(amount)
{
   if (amount == undefined) amount = 0;
   var last_balance = parseInt(document.getElementById("balance").innerHTML);
   document.getElementById("balance").innerHTML = last_balance + amount;
}
</script>
<p>Оплата: <span id="balance">0</span></p>
<input id="balance_amount" type="text" placeholder="Введите число" />
<button onclick="increase_balance(parseInt(document.getElementById('balance_amount').value))">Увеличить баланс</button>