Factory buckets will usually be around 40 lbs each or about 20 lbs lighter than a factory split bench seat which in itself is 100 lbs.
One piece front bench seats will be a tad lighter since the split portion reinforcement is not in the framing works.
A100 low back reclining van seats weigh in at 22 lbs each and can be lightened a couple of pounds from there. Plastic high back race seats depending on what make and design can be as light as 8 lbs each.