Bulk Update app will help you change your product price very easy and fast.
You can change the price of specific single variant, change price of all product in a collection, or even change price of all product in your store.
You can change it with percent amout or fixed amount.
And it still work in shops with thousands of products ! (basically it will take about 1s per product).
If you have any ideas to improve this app, please send me your I ideas to firstname.lastname@example.org
**** Update 20/3/2016: add Increase, Decrease operation, add field label
**** UPDATE 23/5/2016: Fixed changed price whole store and collection issue in review report, hope you enjoy it!
**** UPDATE 20/6/2016: Fixed charging issue!