@dan, ok I'm not saying this is pretty but it works. I'm using jQuery to grab the amount store it in a variable, then use that in the location string.
on_sent_ok: " my_amt = jQuery('#sponsor_donation').val();"
on_sent_ok: "location = 'https://www.sandbox.paypal.com/cgi-bin/webscr?business=contributions-facilitator@teamjulianfoundation.com&cmd=_xclick&amount=' +my_amt;"