I have made a component that installs via the Joomla installer. It can update prices by category, by shopper's group with Virtuemart bulk price options, based on the default shopper group price, by giving a percentage and choosing an action.
It is inspired by on Bureau Van Vliet B.V. VM Category Discount component (bureauvanvliet.com).
Tested on Joomla 1.5.22 and Virtuemart 1.1.6.
All you have to do is install and:
1. Choose the category
2. Choose the action (-/+) and the percentage from the default price
3. You may add the quantity that the price starts
4. You may add the quantity that the price ends
5. Choose the shopper group from the list below
6. Click on Change prices to products
Download:Download via Nitroweb