Wood and foam is cheap....
Seriously though, there is actually quite a bit you can do with a wood base, some fibreglass and foam.
If you can fabricate metal then you do a lot more. It all depends on your budget. All of the aftermarket seats I've seen to purchase have been high end.
Salvage yard might also be a good option.
1981 CB750K with 900 cams
90K KM's, rebuilt head, rebuilt carbs, upgraded valve stem seals
My wife's recipe website that I'm trying to help promote:
Strawberries for supper. Yes, I am a lucky man.