Question: Why is the upper sirloin is generally more expensive than the lower sirloin?
About the article "Steak", section "Sirloin steak".

Candidate passages:
The sirloin steak is a portion of the sirloin part of the cow which is near the cow's rear, the nearer the meat is from the rear, the tenderer it is likely to be, like the rump steak.
Although there are two parts of the sirloin: upper sirloin is generally more expensive than the lower sirloin because it is more tender.
Sirloin steaks are often served at steakhouses and are often accompanied with French fries and vegetables such as broccoli, carrot or peas.
They are usually cooked by grilling or frying.
Top sirloin is a cut of meat from the primal loin, subprimal sirloin, of a beef carcass.
Top sirloin steaks differ from sirloin steaks in that the bone and the tenderloin and bottom round muscles have been removed; the remaining major muscles are the gluteus medius and biceps femoris (top sirloin cap steak).
Some American butchers call a thick top sirloin steak a chateaubriand, although the French reserve that term for a more premium cut from the tenderloin. 
The sirloin steak is a steak cut from the back of the animal.
In US butchery, the steak is cut from the rear back portion of the animal, continuing off the short loin from which T-bone, porterhouse, and club steaks are cut.
The sirloin is actually divided into several types of steak.
The top sirloin is the most prized of these and is specifically marked for sale under that name.
The bottom sirloin, which is less tender and much larger, is typically marked for sale simply as "sirloin steak."
The bottom sirloin in turn connects to the sirloin tip roast.
In British and Australian butchery, the word "sirloin" refers to cuts of meat from the upper middle of the animal, similar to the American short loin. 
"Sirloin" is almost certainly not his real name, but nobody knows what else to call him.
He was on Perkton's community softball team before George and his friends joined, and used to be in a heavy metal band.
Nobody's terribly sure of his profession or background.
His closest friend in the strip is Dave, as he has more in common with him than anyone else.
In early 2008, after being prodded by George, Sirloin revealed that he used to be in a heavy metal band called Flatulator.
Dave convinced Sirloin to take up his guitar (named Guitaron) again, and upon doing so, Sirloin again grew a full head of hair.
So far, Sirloin has kept the long hair. 
  Chateaubriand steak: Usually served for two, center cut from the large end of the tenderloin.
Sometimes it's extra thick top sirloin.
Chuck steak: A cut from neck to the ribs.
Cube steak: A cut of meat, usually top round, tenderized by fierce pounding with a mallet or mechanical blades.
Filet Mignon: A cut from the small end of the tenderloin; the most tender and usually the most expensive cut by weight.
Flap steak: A cut from the bottom sirloin.
Flank steak: From the underside.
Not as tender as steaks cut from the rib or loin.
Flat iron steak: A cut from under the shoulder blade.
Hanger steak or (French) "onglet": A steak from near the center of the diaphragm.
Flavorful, and very tender towards the edges, but sinewy in the middle.
Often called the butcher's tenderloin or hanging tender.
Popeseye steak: Thinly sliced rump steak, originating in Scotland and available in the UK.
Ranch steak : A chuck steak usually cut no thicker than one inch, 10 ounces or less, and trimmed of all excess fat  Rib eye steak, also known as Scotch fillet, and Entrecôte: A rib steak consisting of the longissimus muscle and the spinalis or cap.
This comes from the primal rib used to make prime rib which is typically oven roasted as opposed to grilled as is typical with rib eye.
Also known as a Spencer Steak.
Round steak, rump steak, or (French) "rumsteak": A cut from the rump of the animal.
A true grilling steak with good flavor though it can be tough if not cooked properly.
Sirloin steak: A steak cut from the hip.
Also tends to be less tough, resulting in a higher price.
Outside Skirt steak: A steak made from the diaphragm.
Very flavorful, but also rather tough.
Inside skirt steak: A steak from the flank or bottom sirloin similar in appearance but more tender than the outside.
Strip steak, also known as New York strip: A high-quality steak cut from the strip loin, a muscle that is relatively low in connective tissue, so it is particularly tender.
T-bone steak and Porterhouse: A cut from the tenderloin and strip loin, connected with a T-shaped bone (lumbar vertebra).
The two are distinguished by the size of the tenderloin in the cut.
T-bones have smaller tenderloin sections, while the Porterhouse – though generally smaller in the strip – will have more tenderloin.
T-bone and Porterhouse steaks are among the most expensive steaks on a menu because of the large individual portion size.
Tri-tip steak/roast: Also known as a Triangle Steak, due to its shape, it's a boneless cut from the bottom sirloin butt.
Types of beefsteaks.
Several other foods are called "steak" without actually being steaks:  Beef tips: Small cuts of high or medium quality beef left over from preparing or trimming steaks, grilled and served in a manner similar to the cuts they were taken from.
Common as a "budget conscious" option for those who want to eat steak but cannot afford (or cannot consume) a whole steak.
Salisbury steak: Not a steak, but rather a burger from ground beef made with onions, usually bread crumbs, and occasionally mushrooms.
Also known as "Hamburger Steak" or "Minute Steak" (due to its shorter cooking time).
It is the least expensive "cut" of steak, usually because it is made of lower grade meat.
Steak tartare or tartar steak: Finely chopped raw fillet of beef, onion, parsley, capers, a hot sauce (usually Worcestershire) and raw egg.