Magento’s layered navigation is an extremely powerful navigation tool. Customers can use it to narrow down a category collection and filter only results they are interested in.
If layered navigation has been enabled users can filter by price. By default this price is calculated by Magento automatically by equalising the product ranges. If you want to override this without hacking Magento, you can change this setting via the Magento admin panel.
System -> Configuration -> Catalog -> Layered Navigation
You will then see a dropdown called “Price Navigation Step Calculation”. Change this from automatic to manual and you will then be allowed to change the navigation step and the maximum number of price intervals.