I wrote this module for a client and figured other people might be able to use it as well. Very simple and straight forward, also very risky. This module should ONLY be used if you have both List Price and Sell Price set for each product.
First go to Store Admin > Batch Price, select what operation you would like to perform on the products sell price and input a percentage change.
Basically the module will take the list price as the base price of the product, multiply it by the factor input and update all the products in your store with the new price. As you can see, this is a very "dangerous" operation thats why List price should always be set because I don't change it. So if anything goes wrong, you can reset your sell prices back to the list prices.
The good thing is - that it works! More features could be added later.