It's (it IS) worth paying two dollars for.

Unfortunately, neither is correct. Say "it is worth paying two dollars (for)".

Neither. "Worth" is an adjective, so you would say "It is worth 2 dollars."

Many thanks!

It's worth paying... That's the construction. You can have '...it worth....' but those words have to sit in a phrase or expression similar to 'I think it best' (I think it is best). I think him foolishly vain. (I think that he is foolishly vain) So, I consider IT WORTH the price. (I consider it is worth the price). But apart from this construction we need the IS. It IS worth paying.... It IS worth the money. It IS worth the bother. I consider IT WORTH thinking about ! It IS worth it !