The Transtrem on the Steiny guitars allowed the player to lock in different tunings for all 6 strings with just a quick flick of the trem, as well as functioning as a whammy bar. IIRC, there are actually one or two Van Halen songs which cannot be duplicated completely accurately without one (though it can be faked) as they were recorded by Eddie using the Transtrem to change the guitar's tuning mid-song. I'm not sure if the bass Transtrem does the same thing -- I've never played one. The downside to the Transtrem was that in order for it to work, the strings had to be EXACTLY the right length -- I think they called that particular type of double-ball strings "calibrated," but I can't really remember as it's been years since I even thought about one of these.
Prices have gone up. I think I saw one of these go up for sale in the $3k-3500 range a year or so ago. But mainly it's just rarity -- Transtrem-equipped basses are VERY rare, so prices are mostly determined by whether or not there is someone willing to pay it on the belief that they won't have another chance to get one.
At one point when I was playing fivers exclusively, I actually considered paying $4500 for an XL-5W from Ed Roman!!! Thankfully, that madness (on my part) passed and someone else bought it, but the outrageous price was dictated by the fact that there are fewer than 70 of them that were ever made, and there are no other basses that can produce that same tone due to the unusual composite they used. In the past 5-7 years or so, I have only seen or heard of two of them going up for sale, with one of those being the one sold by Ed Roman.
FWIW, my XL-2 cost me around $1k new in 1988. In terms of inflation, they haven't gone up all that much in value. In fact, I'd bet that they are actually worth less in 2009 dollars than they were in 1988. I might have been able to buy a 1960s Fender for $1k back in 1988.
