Pah!
I say the simplest solution would be for a fresh crossbow to not come with full ammo. Make it come with, say, 30 out of 40 bolts. It's meant to be a balancing factor for having such a powerful weapon, so let it act out it's role.
Besides, I personally have never really needed to buy ammo anyway. I use the 9mm as my main, and handcannons for close combat, so the crossbow is only for scrakes, distant fleshpounds, and patty (and the rare cases where there's a huge lineup). Therefore, I don't use a lot of ammo, and what I do is easily made up by grabbing an ammo box. They give you a whopping 10(!!!!) bolts, so it's definitely worth it. (Though it sucks when I'm saving an ammo box for later when I need it and a teammate grabs it in the beginning of the wave).