Extracycle is ~$500, allows 100-200lbs depending on rider, and allows about as much volume as a sedan.
A Burley cargo trailer is ~$400 new, allows 100lbs (or less) depending on rider, and allows about as much volume as a sedan.
A B.O.B. is ~$300 new, allows 70lbs or less, and allows about as much volume as a sedan.
A used kiddie trailer is $50-250 depending on make and model, mostly allows 100lbs as a max, and allows as much volume as a sedan.
So the real issue is not price IMO... it's how much density you need to haul. I'm just not strong enough to manage more than 50-60lbs of dead weight, so for me a B.O.B. might do the trick if I needed a massive volume increase. An Extracycle is just overkill. For me, the point where one would be useful is if I had kids. Without mobile cargo, I'd end up with about the same maximum weight limit I have now.