is it ok to weight potatoes after they have been cooked

I personally would weigh them raw.

How you cook them affects how much the weigh. I would guess that boiled potatoes would have extra water weight because you cooked them in water, while roasted/baked potatoes would lose a little bit of their water and weigh less. Haven't actually tried though.

because i thought when i weight them raw when you poor out the water you would lose some of the potato ( are the cooked and raw spuds close in weight )

Whether they gain or lose moisture (and therefore weight) through cooking depends on how you cook them.