well for starters a sump is an extention of your tank that runs inline with it. its used to keep all those unsightlly things out of your tank and in the sump such as your protein skimmer, bio material ect ect. also its a good spot to add stuff to your water without direct to your tank.
depending on how intricate you want the price varies. i made my own out of a 10 gallon tank and made partitions in it for water to flow channeled. gravity drops the water from a overflow syphon and a power head pumps it back up to the tank. also its a good source of water flow into the tank for more current.