Ye Olde Inn

For each combat die rolled in an attack with the crossbow, subtract one gold coin from your character sheet.

That’s it! This is an easy way to keep the crossbow’s OP in check, and takes into consideration a Hero using courage, heroic brew or potion of strength, etc. with number of dice. And if things get dire and the Hero only has one gold coin, maybe they can fire off a single die desperation shot!

How many shots do you fire per game?

The idea is nice, though I guess much testing is needed for the exact price

Why not just up the monsters, once the heroes have a crossbow, you start to give the monsters an occasional crossbow, too.

If we consider a "tough" quest to be about 37 monster body points, sure, you can account for all the times the Crossbow misses, but chances are its going to do some damage, and so let's say you get great rolls, you'd only need about 13 shots to take them all out. That's pretty reasonable. Of course you can argue all over the place in terms of "realism." Supposedly in history there were archers who carried a lot of arrows into battle and for the big battles, they had wagons resupplying them. Instead of imagining that the Crossbow has magical infinite ammo, I just assume the Heroes wisely brought just enough bolts as they would need on this particular quest.

But as for being OP I can see putting some restrictions on it. I'd start with making it so it can't hit the four diagonal adjacent squares (always has to have another square in between, just like adjacent) and I'm cool with it. You could also transform it into a two handed weapon (for now it functions as one handed) meaning you can't use it with a Shield.

You could even go as far as the Japanese edition does in which it can only be used by the Elf (they also restrict the Elf's armor options to only the shield so I guess that wouldn't work with the above nerf).

Other than the diagonal thing (which I think was an oversight on the part of the designers of the NA rules) I don't think it needs nerfing, but if you do, good luck. It firing gold coins as ammo (like the Lone Ranger's Silver bullets... a symbol of justice) is a very creative idea I would agree.

"Toss me some of your coins, I need to get off the shot!" "Get 'yer own mate, I've got my own problems!"