I can't give you any up to date information, but I bought one new in California in 1983 for $350.00. It was my understanding that it was one of the first and I bought it new. Mine had no springs whatsoever and had a hitch similiar to the Bushtec. I pulled mine with an 81 Wing more than 50,000 miles and the only problem I had during that time was the wheel bearings had to be replaced once. Mine looked like the base model and I wish I still had it.
To be honest, even with no springs and around 20 lbs in the tires, I never knew it was behind me. The only time you did was going slow and hitting a pothole or lip in a driveway. I always had to have something in it to hold it down I don't think it weighed 100 lbs. It was narrow enough to take into your motel room and you didn't have to unpack it, just open the lid like a suit case.
I purchased from an estate sale 3 yrs ago a Wingliner 20 cu ft model that was mfg'd some 20 yrs ago (per Motorvation)but never assembled. We have pulled it about 8,000 miles and no complaints. Never know it is there.