Illustration: Mr Cuddington
The effect doesn't stack with itself. You don't get to offer half of half price if you play two copies of Building Contractor at once. This is because each one gives a discount based on the marked coin price, which is the build price number printed on the card.
If you play two copies, you simply have two different ways to offer half price.
If two different players use Building Contractor at once, other players have two options to choose from.
The same goes for other effects that give half price. You can have multiple ways of accessing half price builds, through different abilities such as Renovation, but they can't be combined together.
The card rule doesn't specify where the card that players wish to build must come from. It could be their hand, the Market, or it could be their Showcase super attraction.
Yes, that is a body in the foundations. Whaddya expect for half price?