I am often asked how much someone should spend on a Bell-Matic today. 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, on the left is a 1969 newspaper ad for a Canadian seller. Note this 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.
Next is an ad showing two 4006-603x's and a 4006-602x. I believe that this ad is circa 1969-70. The Bell-Matic on the left has the (now) very popular "coffin case" bracelet. Next to this ad you can see the price for a high-end diver of that time.
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.
