I don't the exact events, but I know what needs to be done.
You need to use variables for this. VCE would probably work, but what do I know.
With variables, you would have a variable called 'points' or something, and you would have it set up so everytime a player gets a kill, they would get a point. When you go to buy a weapon, it checks to see if you have enough points to buy the weapon. You would also set the weapons cost with variables. If you have enough points, the amount of points that the weapon costs, will be taken from your point total.