If fast food prices were cut in half ...

If fast food prices were cut in half today, most people would not purchase any more burgers.  How many can they (we) eat?  
 
We do not know how far below most people's "maximum buy line" prices may have sunk during decades of down-drifting minimum wages -- plausibly meaning today's prices could rise 25% higher and people might buy just as many; 50% might cut into business.  We DO know today's minimum wage is $3.50 an hour below what it was in 1968 -- we DO know per capita income has near doubled over the same time frame.  (See chart below.)  It is perfectly plausible that fast food prices began dropping deeper and deeper below the "maximum buy line" decades ago!

Look at Wal-Mart: 7% labor costs.  DOUBLE Wal-Mart's average wages ($10 to $20 an hour) and ADD health benefits, paid vacations, etc., and prices might go up 10% (7% + 3%?).  If Jimmy Hoffa's Teamsters were in there that would have occasioned long ago.

Look at most of the Americans you meet working on less than specific training required jobs (like x-ray tech): they are embarrassed.  They are earning $400-$500 a week.  $500 is today's median income.

Look at the official federal poverty line: 3 X the price of an emergency diet (dried beans only please; no expensive canned) -- a formula from the mid-fifties = $20,000 poverty line for family of three ($400 a week).  Realistic minimum needs line based on table 3-2, p. 44 (after adjusting for inflation) in the MS Foundation book Raise the Floor works out to more like $50,000 a year for family of three if it has to pay its own medical insurance ($1,000 a week!).  HALF OF TODAY'S AMERICANS EARN HALF THAT POVERTY LINE OR LESS! 
  * * * * * * * * * *
double-indexed is for inflation and per capita income growth (2013 dollars):  

yr  per capita    real     nominal  dbl-index   %-of
68    15,473    10.74      (1.60)     10.74      100%
69-70-71-72-73              [real, low point -- 8.41]
74    18,284      9.47      (2.00)     12.61          
75    18,313      9.11      (2.10)     12.61
76    18,945      9.44      (2.30)     13.04        72%
77                                                               [8.86]
78     20,422     9.49      (2.65)      14.11
79     20,696     9.33      (2.90)      14.32
80     20,236     8.78      (3.10)      14.00     
81     20,112     8.61      (3.35)      13.89        62%
82-83-84-85-86-87-88-89                          [6.31]
90     24,000     6.79      (3.80)      16.56  
91     23,540     7.29      (4.25)      16.24        44%
92-93-94-95                                                [6.51]
96     25,887     7.07      (4.75)      17.85
97     26,884     7.49      (5.15)      19.02        39%
98-99-00-01-02-03-04-05-06                      [5.97]
07     29,075     6.59      (5.85)       20.09
08     28,166     7.10      (6.55)       19.45
09     27,819     7.89      (7.25)       19.42        40%
10-11-12                                                      [7.37]  
13     28,829     7.25      (7.25)       19.32        38%
 * * * * * * * * * *
A $15 minimum wage shifts 3.5% of income from the 55 percent of the workforce who garner 90% of income to the 45% who get 10%.  $8,000 average raise X 70 million (45% of 140 million + 5% at minimum now) = $560 billion out of $16,000 billion GDP.  Somebody challenged me that raising Walmart prices 10% (only 3.5% at $15 min) would take $26 billion from the poorest consumers.  I pointed out that with an extra $560 billion they wouldn't be so poor. :-)