This may confuse you, but you may run into this corner case so I'll just say it anyway: Quarry lowers prices for action cards only and you can put it in play and play more actions (like Upgrade) later because of the Black Market card which gives you an extra, intermittent buy phase.
So if you play: Village, Black Market (put Quarry in play, don't buy anything out of the Black Market deck), Upgrade, you can Upgrade an Estate (still costing $2, Quarry has no effect on it) to a Market (now costing $3 due to Quarry).
This is a very rare case though.
Another tip: For every Highway in play, costs (for every card) are lowered by $1. This means that if you Throne Room or King's Court a Highway, it will still only lower the costs by $1, not $2 or $3. The same goes for Princess. Bridge is different and if you King's Court a Bridge, you actually lower all costs by 3 * $1 = $3.