Calculating which one you should buy will require some number crunching. This is a situation where you will be glad if you have remembered to bring a calculator to the store. (If you have forgotten, but you have a cell phone, it probably has a calculator function buried in the Tools or Applications menu.)
First, check the percentage. Two typical ground beef packages at the store where I shop will be "96% lean" and "85% lean." This means that the first package contains 4% fat, and the second package contains 15% fat.
(A word on fat: fat lends flavor to the dish, and is necessary if you plan to make a sauce. But for the most part, when you're buying ground beef, you will end up pouring off as much fat as you can. I'm going to assume, for the simplicity of the calculations, that you do not want to use ANY of the fat, and that you will be pouring off and throwing away all of the fat. Obviously this isn't completely realistic.)
Let's say that the first package costs $4.79/pound and the second package costs $3.29/pound.
4% of $4.79 is .19, which means that you will throw away 19 cents per each pound of the extra-lean meat. However, you will be paying $4.60 per pound for the meat itself.
15% of $3.29 is .49, which means that you will throw away 49 cents per each pound of the lean meat. However, you will be paying $2.80 per pound for the meat itself.
(Remember how you used to complain in school that you would never need to know algebra? Yeah. Sorry about that. As a refresher, to calculate a percentage as we did above, it's 4.79 x .19 and 3.29 x .49.)
As you can see, this is an odd case where it is more frugal to buy the cheaper meat, which technically requires you to throw more money away. But the overall savings is still considerable - in fact, this meat costs about half as much as the "fancier" kind.
It's never good to throw money away, but the differential here is great enough that it's cheaper to do so. Think of it this way: even the cost of the cheap ground meat AND the fat is still cheaper than the cost of the expensive ground beef WITHOUT the fat.
This should also serve as an excellent illustration of one of my pet phrases, It's Important to Do The Math. For one thing, the price of ground beef varies so widely, you will probably have to do the math yourself each time, until you get a feel for it. For another thing, the results are completely counter-intuitive.
So why is the leaner meat more expensive by pound? Here we get into the murky areas of frugality and food pricing. More affluent (read: wasteful) customers prefer not to have the mess and stigma of all that fat in their meat, and they are presumably willing to pay a premium for the privilege.
But you and I know better now, don't we?!
Photo credit: Flickr/ilovebutter