I've downsized my collection and am down to 3.
100-150 is about the going rate for the SB310 (the basic active model), unless prices have jumped up recently. The one at 100 seems OK, the ones at 250 I reckon are overpriced. The one at 170 is an SB100, I'm not sure about these but they may have been made after the original SCG Nanyo company went bust and production got moved to other locations. To be fair, all the SB100s I've seen have been good quality.
Mick the Greek has had a few of these, and Happy Jack is the font of most of the knowledge - there's a thread called Obscure Musical Backwaters in the Bass Guitars forum which contains most of Jacks research (plus a load of photobucketed empty spaces!)