The Rare Cyber Twin Dragon from Duelist Pack: Zane Trusdale (DP04-EN011) is worth approximately 50 cents to a dollar. The Super Rare Cyber Twin Dragon from Cybernetic Revolution (CRV-EN035) is worth...
It depends on the number of the card. Some of them are worth more than others. The common ones are worth from 60 cents to about a dollar twenty. A super-rare one was worth around twenty dollars....
Not much really. It is out of print, but still mass produced. A collector outside the US/Canada might want it, but probably not pay more than a £1 or so for it.
The price of a card will almost always depend on the rarity of it. As Stardust Dragon was printed in multiple rarities, the answer to this question cannot be determined by the information presented.