First off, that's about the most common question ever in the reloading forum (so I moved it here). Everyone asks that as they contemplate getting into the side hobby of reloading, and each person's answer to the question is slightly different. The best answer is based on a combination of factors:
1) Cost: How many rounds are you shooting per month, and what kinds of rounds? Rifle ammo these days is ranging from anywhere between about $0.12 a shot (surplus 5.45x39mm) to a dollar or so per shot (most common hunting cartridges) to many dollars a round (the big Weatherbys, large dangerous game rounds, anything obsolete or uncommon). Pistol ammo varies all over the place. 9mm can still be had cheaply enough that getting into reloading just to shoot a box or two of that per month probably doesn't make sense. On the other hand, I shoot a lot of .44 Special some years (like maybe 8-10,000 rds.) No way I'm paying factory prices for that...if I can find enough at all.
Just like factory ammo, reloaded ammo costs vary all over the place. At the bottom end, in pistol rounds, cast lead bullets might be 5 cents a piece, a penny or two for a charge of powder, about 3 cents for a primer, and range-pick-up brass is free, so you're shooting for about ten cents a shot. (You can cast your own bullets, too, for "free" if you scrounge lead ... but that's a whole 'nother ballgame.)
On the other end, if you're buying premium rifle bullets and loading large or unusual, or high-precision cartridges, the bullet itself can be over two dollars a piece for some of the crazy target and long-range stuff, a new brass cartridge case might run you over $5, etc. But if you look at buying that same loaded cartridge, you still might be saving half or even more of the cost.
So let's just say "half." You might reasonably expect to load common ammo for about half the cost per shot, depending on a lot of factors.
2) Time: Time is money, sometimes. Do you work a lot of hours, have some disposable cash, but have little spare time to shoot? You're probably better off buying ammo, and using your valuable spare time for practice. Do you have more free time than spare cash? Reloading may make sense right there.
3) Ammo factors: Is what you like to shoot always on the shelf at the local store, or always in stock at the online retailers? Is the price stable? Are you satisfied with the factory ammo? Is it accurate enough for your needs? (Do you have a sneaking suspicion you could do better with ammo tailored to your rifle?) Are there other bullets you'd like to try that aren't available in factory loads, or only in super expensive premium ammo? Other power levels you'd like to be able to use? Want some reduced loads for plinking or teaching a young person or your wife? Want to try cast lead bullets? Reloading your own ammo gives you a benefit here that isn't measurable with cash. You can make things that cannot be bought at all. That's valuable, if you want it.
4) Initial costs: Again, how many rounds per month? You can buy a single-stage press kit from Lee for something close to $100 and be churning out great ammo right away. Now each 100 rd lot is probably going to take you an hour or two to make. If you shoot 300 rds a week, that might be way too much time investment to justify, week after week. If you shoot 100 rds. a month -- maybe that's not so bad. If you shoot 100 rds a week+ and time is somewhat at a premium, a progressive press is a better idea, as you'll be able to run off 400+ rounds in an hour. But the costs to get into a progressive are something more like $300-$600+ (and way up, depending on features and speed). If you're shooting a lot, the savings per round will pay you back at some rate (depending on all these factors).
5) The "more is more" effect: Most of us who reload find that we shot some when we were buying ammo, but then shot a LOT more after we started reloading. Shooting 3x the ammo, even at half the cost, is going to cost you money.
6) Can you really do it? I don't mean physically. Of course you can. But can you do it practically and safely? Do you have a place to dedicate to a high-precision activity that can't be disturbed, can't be cluttered with random household junk, is free from distractions and interruptions that could cause you to make a serious error? Do you have time you can set aside where you aren't going to be intruded upon, called away, watching TV, or otherwise subject to diversions of your attention from the matter at hand? Not to scare you or dissuade you, but the results of an error are NOT good. (
http://www.thehighroad.org/showthread.php?t=595480)
As I said, there are many discussions on this that can shed more light on the question:
Our sticky on the subject:
http://www.thehighroad.org/showthread.php?t=18835
A couple of recent posts:
http://www.thehighroad.org/showthread.php?t=596157
http://www.thehighroad.org/showthread.php?t=595540
...and dozens more...