Of course, the answer is "It all depends." You need to consider the apparent condition of the watch, the reputation of the seller, the location of the watch, and whether it looks like someone has mixed different Bell-Matics into one. One thing I can share with you are some original prices from the days when these SEIKO's were new. For example, here is a 1969 newspaper ad for a Canadian retail seller.
(Click on the thumbnail for a larger image)
Note this advertisement is for the now very rare 21J model and the price is Canadian $139.95 for stainless steel and $149.95 for the "gold colored" case and bracelet. If we do some calculations for inflation, it soon becomes clear that the SEIKO Bell-Matic was one of the most expensive models in SEIKO's line of watches.
For comparison, some of the SEIKO dive watches of that day that are now in such demand with collectors actually cost
less than the Bell-Matic when both were new. As an example, here is a German ad from 1973 that includes a 4006-603x (top row, third from left) and just to the right of the flowery asterisk, what looks like a 6105 diver (one of today's most desired vintage SEIKOs). The Belle is DM320 and the diver is DM310. (Click on the thumbnail to expand the image and, when it opens, you can "zoom" that larger)
Regardless of prices paid today for these vintage SEIKO's, one thing we can say for sure: Bell-Matics were an elite, high-end offering at that time. No wonder so many of us today have fallen in love with them.
