I like the slot machine idea. I think all the weapons should be purchasable off the walls. (maybe excluding the ray and waffle) But they should be higher priced of placed in rooms far away from the starting point that it makes it more economical to make a run at the box before venturing out to buy weapons off the wall.
Also, i think there should be a universal ammo area where you can buy ammo for any gun. It could be set up where you can only purchase a particular type of ammo once a round, then it is available only once the next round and so on and so forth. Something like that could boost teamwork or add a level of sabotage...