attempt to dynamically populate product price

i'm trying to pass from my custom post type, the value of a trip to the form to collect a trip deposit. i'll eventually be calculating 20% of the trip rate, but it's not passing this data through. guidance appreciated.

add_filter('gform_field_value_trip_deposit', 'populate_post_trip_deposit');
function populate_post_trip_deposit($value){
    global $post;

    $trip_rate_value = get_post_meta($post->ID, 'trip_rate');
    return $trip_rate_value;
}

sorry. i will try to get something up to demonstrate tonight. basically on this page, i want to have a button, 'book your trip'. based upon the value of the trip rate, calculate 20% of that and pass that price dynamically to the form page where they can specify the quantity (number of travelers) and book through paypal. the Query string works, but i want the value hidden so users can't edit the value.

OK I think I have a solution for you, hopefully this works in your situation based on my example I made here. I can email you the XML file for the form created as well, if you'd like.

So we have this form here:
http://grab.by/d1ci

The Trip ID field is set to be admin only and to be populated dynamically (this is what will control our pricing). The three trip prices I have conditional logic on to only show when the matching trip id is passed.

So you can see the result here, each of the three trips shows the appropriate price, so no one can change the price on you for that particular trip, unless they want more than 1 of course.

http://dev.outerhavenstudios.com/trip-form/?trip_id=Trip%201
http://dev.outerhavenstudios.com/trip-form/?trip_id=Trip%202
http://dev.outerhavenstudios.com/trip-form/?trip_id=Trip%203