The seem to range from $650 US for BGN early model to $1500 US for a black late model, factor in $150 for postage though and there you have $800 -1650 which sounds about right.
Mine is a 1977 model and it was in pretty good condition when I bought it about eight years ago. I paid about $1500 back then, and they still seem to max out around that price, so I guess in real terms they are getting cheaper. There is a big difference between say a mid-1970s model and an early 1990s model.