Here is a route planning plugin that lets you calculate estimate cost of trips on your site. Car Route Planner for WordPress supports your own formula for price calculations. It allows for worldwide routing and lets you edit routes via drag & drop. You can also easily select start/end points on the map.
The plugin has minimal and advanced modes. It supports metric, American, British imperial systems and various languages and currencies. Webmasters can customize which items are shown in the results.
Car Route Planner is available as a free download. More info is available here.