I can see why you say that, but at the same time I'm trying to throw the price out of whack. If a crossbow and unlimited quarrels is 350, I'm trying to find an even price that sould add up to such.
However I can also see the intrigue in limiting the quarrels if it's 5 gold per dart, that would be between 50 - 75 gold per quest (assuming the hero fires the Crossbow 10 - 15 times a quest), maybe I'm overestimating. But I could envision heroes starting to use the crossbow sparingly towards the end of the quest when they get low on darts.
The only problem I can see is losing count of how many times fired. For this I guess I'd use tiles to keep track.