I think my explanation jumped to combos to quickly. Here's a simpler idea. Highway is roughly a cantrip which gives one coin per buy used this turn.
Yes, this. Forget about combos at the start, just look at how many buys you use.
Bridge is +1 buy, +$3 if you buy 2 cards, (which it in itself allows you to). That's already better than a descent $5 card would be. If you buy 3 cards this turn, Bridge is +1 buy, +$4.
Princess is similar to Bridge. Sure it's a Woodcutter if you only buy 1 card, but it gives the +buy, so assuming you buy 2 cards, it's now +$4. If you buy 3 cards, it's +$6!
Highway is more comparable to Peddler than Bridge or Princess. +1 card, +1 action, +$1 would be a descent $4 card. There are many variations on this in the game; most of them cost $5 because they have a small advantage over that basic card. Market gives a buy. Treasury can go back on top of your deck for next turn. Junk Dealer lets you trash a card. And Highway gives 2 separate advantages: it's $1 for each card you buy, instead of just $1 total; and it has a number of combos such as being able to Remodel a Copper into a $5 card after playing 3 of them, or being able to Ironworks a $5 card after playing just 1 of them.
Quarry - even if it were just a Gold-for-actions, that alone I think would be worth $4. In most games, most of your buys are going to be action cards. That's a Gold for $4 instead of for $6. Of course, it has the drawback of being a Copper when you want to buy a Victory card or Treasure. But on top of that, it's +$5 if you're buying 2 action cards. +$7 if you're buying 3 action cards.