Hello, I think there is a very serious bug.
Example: Add a product to the basket which costs 20 euro. Then add a product whick costs 25 euro. After adding the second product, if you check the basket, the total value is 50 euro because the price of the first product changes from 20 to 25 (equals the second product).
After testing my site, I found out that your module was the problem.
Just to make sure, I made a new site on my localhost by installing the latest version of joomla (2.5.9), the latest version of virtuemart 2.0.18a and just your extension. The same problem occurs, so you will be able to test it.