I am not sure if I understand you right but the easiest thing to do this is:
- do not import the prices in the shop (set them to 0 instead)
- search for “price” in the app/design/frontend directory and adapt the phtml-file as you need it.
- do the same with app/locale/*/template directory
- Test your system thoroughly.
You will need to do a few things more e.g. remove the taxes and totals in the cart.phtml but I am sure you will find the things you have to adapt then.