Some mistakes in your code:
1. it should be != not =!
2. redundant
</td>
3. forgot to remove the php code which was supposed to get the current balance from somewhere (database?)
See the modified version below:
<html>
<body>
<table>
<tr>
<td> Collections: </td>
<td> <input type="text" onchange="addNumbers()" id="Collect" name="Collect" value="0" />
</tr>
<tr>
<td> Deposits: </td>
<td> <input onchange="addNumbers()" type="text" id="Deposit" name="Deposit" value="0" />
</tr>
<tr>
<td> Current Balance: </td>
<td> <input type="text" id="CurBal" name= "CurBal" value="100" readonly="readonly" />
</tr>
<tr>
<td> Balance: </td>
<td> <input type="text" id="Balance" name="Balance" value="" />
</tr>
</table>
<script>
function addNumbers() {
var collect = parseFloat(document.getElementById("Collect").value);
var dep = parseFloat(document.getElementById("Deposit").value);
var curr = parseFloat(document.getElementById("CurBal").value);
var bal = document.getElementById("Balance");
bal.value = (collect + curr - dep).toFixed(2);
}
</script>
</body>
</html>