CiviCRM Community Forums (archive)

Well if you are using webforms to register the participants, and then redirect to a civi contribution form to collect payments, which is what I did, very little custom code is needed. You simply need to calculate the amount they owe and print it at the top of the contribution form. You can also set the default payment amount (still allowing them to change it if they want to pay in smaller installments). This is done with hook_civicrm_buildform();
// Existing intro text set in Civi UI
$intro = $form->_values['intro_text'];
// Append whatever you want
$intro .= 'foobar';
$form->assign('intro_text', $intro);
$price = 500;
$defaults = array('amount_other' => $price);
$form->setDefaults($defaults);