Allow variable "custom" pricing for upgrades using a specific input field name "envira-custom-price". Change to whatever suits your needs.

gistfile1.php

<?php
add_filter( 'edd_add_to_cart_item', 'tgm_cp_add_to_cart_item' );
add_filter( 'edd_ajax_pre_cart_item_template', 'tgm_cp_add_to_cart_item' );
function tgm_cp_add_to_cart_item( $cart_item ) {

	if ( ! empty( $_POST['post_data'] ) ) {
	    $post_data = array();
	    wp_parse_str( $_POST['post_data'], $post_data );
	    if ( isset( $post_data['envira-custom-price'] ) ) {
    	    $cart_item['options']['custom_price'] = edd_sanitize_amount( $post_data['envira-custom-price'] );
        }
	}

	return $cart_item;
	
}

add_filter( 'edd_pre_add_to_cart', 'tgm_cp_pre_add_to_cart', 10, 2 );
function tgm_cp_pre_add_to_cart( $download_id, $options ) {

	if ( ! empty( $_POST['post_data'] ) ) {
	    $post_data = array();
	    wp_parse_str( $_POST['post_data'], $post_data );
	    if ( isset( $post_data['envira-custom-price'] ) ) {
    	    $options['custom_price'] = edd_sanitize_amount( $post_data['envira-custom-price'] );
        }
	}

	return $options;
	
}

add_filter( 'edd_cart_item_price', 'tgm_cp_cart_item_price', 10, 4 );
function tgm_cp_cart_item_price( $price, $item_id, $options = array(), $tax ) {

	if ( ! empty( $options['custom_price'] ) ) {
		$price = $options['custom_price'];
	}
	return $price;
	
}