It is very simple. Use the code below.
Also change the name of the onChange function to something else other than the <select> id itself. In my example, I made the function name as "purchaseFunc()" instead of "Purchase()"
<select name="Deposit" id="purchaseDropDown" onChange="purchaseFunc(this.value);">...
....
<input type="text" name="Amount" id="purchaseTxtBox" value="" size="20" maxlength="10">
....
<script type="text/javascript">
function purchaseFunc(selectedValue) {
var txtBoxAmount = document.getElementById("purchaseTxtBox");
txtBoxAmount.value = selectedValue;
}
</script>
</select>
EDIT: There seems to be a problem in your code. The Id's of the <select> and textbox seems to be same ("Purchase"). I suggest you change them, and make them different.
The OnChange function name also seems to be "Purchase" itself. Please change that also to something unique for the context.