Unfortunately, when the user reaches the payment_success page, the page knows nothing about the payment itself (it's essentially just a static page). However, if you're using the PayPal payment method, you could change the custom return url to pass back the payment_amount. Open up payment_paypal_include.html:
Code:
<input type="hidden" name="custom" value="do;process_payment=1;payment_success=1<%loop config.dynamic_preserve%><%if in.$loop_value%>;<%loop_value%>=<%in.$loop_value%><%endloop%>" />
Just add in ;payment_amount=<%escape_html payment_term_num%> and then on the payment_success page, you should be able to use the payment_amount tag.
Adrian