This is taken from the Q&A on drupalcommerce.org:

http://www.drupalcommerce.org/questions/4417/hide-price-when-outofstock

"I'm only recently started using Drupal and commerce and I try to hide the price field when a product is out-of-stock. The out-of-stock button may remain so, I just want to hide the price. The price should not be disabled, only visually be hidden."

I would think you could go a little further than the initial suggestion.