As a guess...
I would say a 20b under $3k would be a great deal for someone, in the $3ks would a fair starting point, and over $4k is most likely overpriced.
Thad Scott has some 12bs ranging from $2400-$2800, so add 10-15% for a 20b, lands you in the $2640 - $3220 range.
BSA Ltd. had one listed around $3600 and is now gone, and saw one elsewhere on-line for $3400 or so. Not sure where that was or if it is even still there. Both of those were cased, had cleaning tools, and had hand detachable locks.
The article Russell posted from 1980 showed a MSRP of ~US$1170, factor in inflation between then and now, you are at about US$3133 MSRP in 2007 money.
Keep in mind you can find Arrietas starting around $4200 or so. I am not sure what the internal differences truly are between the "entry level" Arrietas and the Crown Sabels.