How to override mage file in magento
Core files in the local folder override the same files in the core folder.
Mage library file : app/code/core/Mage/Catalog/Model/Layer/Filter/Price.php
Your file : app/code/local/Mage/Catalog/Model/Layer/Filter/Price.php
Magento will use app/code/local/Mage/Catalog/Model/Layer/Filter/Price.php
If you have any question please reply. Hope it will work.