If you need to change the Price Range in your Magento catalog go to:
Look for the line:
$range = pow(10, (strlen(floor($maxPrice))-$index));
Change the 10 for a lower / higher number.
Please notice you're editing a Magento Core file.
You will lose the changes if you ever upgrade. You should create the same folder structure and override the file but within the /app/local folder (is there an other way?).
From the Admin
You can also edit the price range from the Admin:
1. Go to System > Configuration > Catalog (under Catalog) > Layered Navigation
2. Change the Price Navigation Step Calculation dropdown from Automatic to Manual
3. Enter the amount you want it to be (e.g. - 50) into the Default Price Navigation Step input box