Hi, your pricebox widget is great! Do you plan to make a chart widget too? That would be awesome
And regarding the Wordpress plugin idea: there's no need for that at all. You can add the code as a custom field and just display the custom field anywhere in your post. If you want access to the custom field from other posts as well, then use this free plugin: "Get Custom Field Values". What is does is it displays the custom field value as a shortcode, so you won't have to mess with code in your editor, and WP won't mess it up either (which it usually does, even if you paste it in the text editor mode)
Have fun