CHARGE $200 PER BULLET, PEOPLE WILL BE LESS APT TO WASTE EM
Like any economic ceiling placed upon a commodity, all this does is ensure that those who possess the commodity in question (in this case, bullets) are those who can afford it, i.e. wealthier individuals. That doesn't mean waste won't occur, as those wealthy enough to sufficiently overcome the level of financial "burden" can still be "wasteful" with the commodity to the extent they have wealth to do so.
In other words, this doesn't work because wealthy people could "waste em" and do what they want anyway.
Unfortunately, the only real option to reduce the number of casualities in situations like these is armed security. Much like planes have air marshals, I think we've reached a point where a large gathering of people will require trained, armed security. Not a beefy bouncer but someone who has been trained to take out individuals like Lanza with a single shot.
Do you realize how difficult this would be to install on a massive scale, such as doing so at every school in every district in every state in the country? Without even considering the cost (which would be ENORMOUS), it may be difficult to find enough individuals with proper training. There are just too many schools.
For example, a quick internet check says there are 500 school districts in the state of Pennsylvania (picking a state at random). That's one fairly large state. There are probably 2-3 individual schools in each district on average, but even if it's just 2, that's 1,000 schools in ONE STATE to put multiple properly trained and armed guards at. Some states are smaller, but still, if you extrapolate it out, that's perhaps 50,000 school buildings in the country.
How many guards are necessary? One probably isn't enough. Say a minimum of two guards at each school, now you're looking at 100,000 properly trained and armed individuals to guard the nation's schools. That's difficult enough to find the right people to trust with being armed and dealing with protecting the nation's children, and again, never mind the enormous cost of paying all these people and making sure they are properly trained and stay that way.