It depends on many factors, of course.
Being in CA you will have to start reloading (if you currently don't) because of the recent law passed. The good news on that is, you can store powder and primers that will work for different calibers. Let's say you have a 30-06 bolt gun and a .308 AR10 or something. In this case, you could actually use the same bullets as well. So by stocking up on primers, powder and bullets, you have the capability to either make a bunch of one kind or half/half.
This also works with handguns (you can use .355 bullets for 9mm and .380 ACP). Or you could use Bullseye powder in .380, 9mm, .40 and others.
For maximum efficiency you would prioritize your spending on buying long guns that can use the same powder and even the same bullets (though typically an ideal bullet for one gun is less than ideal on another).
If you buy factory ammo, just have a decent lower limit on how much you will have on hand before purchasing the next gun. A good idea is to buy a 2nd gun in the same caliber and possibly one that uses the same mags.
Finally, look into barrel conversions. My XD40 can be turned into a .357sig or a 9mm just for spending $155 on a barrel (The 9mm does require different mags). This basically gives you 2 guns at a cheap price.