Gravity Forms - product field "user defined" price max value

functions.php

// Form product field validation - max price of £500 
// gform_field_validation_3_9 the 3 is your form ID and the 9 is your field ID
add_filter( 'gform_field_validation_3_9', 'price_custom_validation', 10, 4 );
function price_custom_validation( $result, $value, $form, $field ) {
    //change value for price field to just be numeric (strips off currency symbol, etc.) using Gravity Forms to_number function
    //the second parameter to to_number is the currency code, ie "USD", if not specified USD is used
    $number = GFCommon::to_number( $value, '' );
 
    if ( $result['is_valid'] && intval( $number ) > 500 ) {
        $result['is_valid'] = false;
        $result['message'] = "Maximum donation is £500. Please contact test@test.org to make a larger donation and for further information";
    }
    return $result;
}