How to pass Webform variables to Paypal. Again.

Hi there.

Last day I've spent some time googling around subject. I've found some similar questions and some recipes which is not works for me.

I'm trying to put some additional math to Confirmation message or redirect URL: form field, but without success. Can anyone point me to the right direction?

Here's an example which do not works (base_price and delegate_places are the form component machine names ):

<?php 
$base_price  = $form_values['submitted_tree']['base_price'];
$delegate_places  = $form_values['submitted_tree']['delegate_places'];
$total_price = $base_price * $delegate_places;
print $total_price;
?> 

I've receives zero (0) as the result.

After that I tries put result to paypal submission form like as an example:

...skipped...
<input type="hidden" value="<?php print $total_price;?>" name="amount" /> 
...skipped...

...and it works(!) but passes zero(0) as price amount.

How can I receive webform submission results for Paypal? I feel (actually, i want to believe), that answer is near, but lost my head trying and searching. Help me, please!

Thank you.