The unhelpfull answer would be it's worth as much as someone is prepared to pay. However i'd say (unless it's a rare one, and I don't know much about how rare models are) ?30 is about the most you should pay. If the watch was in mint condition, i.e. no scratches, scuffs, wear of any kind, and working perfectly, it could fetch a little more. Perhaps.
Although you say value is a side issue, I would personally say the most valuble, and distnctive seiko's are the models that have the buttons mounted on the watch face, not the case sides, as is normal these days.