Surely if the ammo was so expensive there would be less demand for it, thus less being produced, thus it becomes harder to come by whether you want to buy it or steal it. The gun crime rate is sure to drop, no?
Well firstly you have to consider the amount of businesses that you are going to affect by bringing in such a measure. Gun clubs, gunsmiths, paid trackers and guides etc. not to mention the manufacturing industry itself. How do you mitigate that? You can't just tell them to find other jobs, many of these people will have nothing else they can do, certainly not to be able to keep up the mortgage repayments.
Then you need to look at the jobs that use guns. We aren't just talking about military and police, in the states you'll find that security will carry weapons, farmers will have weapons, rangers will have weapons. In addition, sport hunting is actually a legitimate way of controlling certain animal populations.
So there are major consumers of guns and ammo who will always keep demand where it is. If you look at the domestic market, actually, people are going to go out, buy a gun and then not use it. It sits in a gun cabinet gathering dust. It isn't like the whole of America goes to the rifle range every weekend. The majority of sales go to business users, be it law enforcement, military or sport. Changing the price of a bullet in a Wallmart isn't going to have a dramatic effect on the criminal aspect of guns in America.
And all of this is a massive aside... Close down every gun store in the US tomorrow and you'd just see increased movement of arms across US borders. The criminals will stay armed. The Mexican cartels would just change their business from drugs to guns.
I think gun laws in the US are mental. However, in the leafy suburbs of Hertfordshire, I don't have to worry that any potential urchin trying to rob my house is going to be carrying a deadly weapon. Until you can convince the people that they are safe from harm, I don't see how you can stop them carrying arms themselves.