ASP coding help for an e-commerce site.?

This form posts to paypal. The amount field in the form at the moment just pulls price from the database and then sends it on, however I want to bring into play discounts so if the person enters the right code '10pct' in the discount field then i can then pass a different 'amount' to paypal. For the time being I want the site to send paypal "4"if the discount code is in place and "3" otherwise...

Discount</td><td>

<input type="text" name="discount" size="15"></td>

</tr>

</table>

<% dim strfinalprice %>

<% If request.form("discount") = "10pct" then 'strfinalprice' = 4

else 'strfinalprice' = "3"

end if

%>

<input type="hidden" name="amount" value="<%response.write ("strfinalprice")%>">

So far all I've managed to do is get paypal telling me to give them a number higher than 0. Anyone here able to tell me what I need to do to fix it?