game #5459986 on frankfurt
In Mix17's last turn, before he (I assume) resigned in disgust, my opponent played 3 individual Highway cards, and also called Royal Carriage on one of them several times. So via one means or another he played Highway something like 6 times. However, the cost of Provinces did not reduce below 5.
game #5460245 on tokyo
After he resigned, I continued against bots, on game #5460245 on tokyo. I found the same thing happened to me; when I called Royal Carriage on a Highway, the cost of the Province only reduced to 7, not 6.
When you call Royal Carriage on a card (or throne room, kings court, disciple, procession) you only copy the instructions above the dividing line.
Some cards in dominion say "while this is in play, ..." explicitly to prevent being copy-able. The original is in play, the copy is not.
In other words... you can copy a Bridge, but not a highway. Well, you can, but you only get the +1 card +1 action twice, not the cost reduction.
I have never noticed that distinction before -- thanks!