Free no, But boy it sure does pay for itself. I usually filter about 15 gal a time mainly because thats what my filtering setup can handle in one batch. A new filter is $ and i have about $30 in parts into it, plus about 45 mins of my time. I would highly recomend at least trying it, and once word gets out that your collecting WMO you will be surprised how many people have some for ya, within a week of having my deuce i was given about 150 gallons PLUS the 200 gallons at the fire house that needed to go. So all in all you save alot of money, especially with fuel at $4 a gallon...
Im preparing to expand my system and seek out sources for oil. right now people just giving me a barrel full every now and then works just fine. and currently i cant keep up filtering alot of oil. once i get a centrifuge my plan is to heat it up and "scrub" about 50 gallons a time. And think about this if your truely serious, a centrifuge can filter down to 1 micron if you do it right, so even at $1000 for a centrifuge, and free oil to use for fuel, it will pay for itself withing what other wise would be 5 fuel ups at the pump!!!! well worth it in my opinion.
Also check the Alt fuels section, LOTS of info there on this topic
Tyler