How much air do you think can flow through this HVAC filter?

I spent this afternoon at a home inspection at a Natomas property I have in contract. As a part of the home inspector's test the central heat and air conditioning system, he popped open the air return grill...and discovered this. Kinda gross!! It would seem to me that this air filter has not been changed in at least a few years? It's a really good idea to change these every couple of months. A dirty filter will slow down the flow of air and ultimately make your HVAC unit work harder (and use more energy) to heat or cool the house. AND they are inexpensive. You can pick up a filter for a few dollars at your local Sacramento hardware store.