All inflation figures were calculated using the U.S. Inflation Calculator.
The effective average buyin for WSOP tournaments has also decreased over the years. It was $1,800 in 1971, $10,584 in 2016 dollars. The average dropped under $10,000 in 1979 ($9,294) and under $5,000 in 1992 ($4,752). After bottoming out at $3,190 in 2004, the average crept up again to $5,638 in 2009.
Then the Big One for One Drop skewed everything. The million dollar-buyin event in 2012 pushed the average all the way up to $21,042. But since it's such an outlier and so many players sell shares of themselves, it's not unreasonable to ignore the $1,000,000 and $111,111 events for comparison purposes. That brings the 2012 average down to $4,106, right in line with every year since then, including 2016, which checks in at $4,407 (down $220 from 2015 because of one fewer $10,000 event and one fewer $5,000 event).
By far the most common buyin all-time and in 2016 was $1,500 (23 of 69 events in 2016; 387 events in the history of the WSOP). There have been 185 total $1,000 events (11 in 2016) compared to 139 $10,000 events (13 in 2016). $5,000 (160) and $2,500 (152) tournaments have been more prevalent, but expect $10,000 events to surpass them in the next few years.
Nine buyins have occurred exactly once (data updated through 2020):
Every buyin had been either a round number ending in zeroes or a string of repeating digits until 2015 when the WSOP introduced a $565 buyin for two events.2
It seems to make more sense as $555 than $565, but maybe they picked the latter because when you're texting, 565 stands for LOL ("laughing out loud") as those are the digits on those letters on the keypad. There were three $565 buyin events in 2016.
The main reason the WSOP stopped having rebuy events was probably that amateurs felt they gave deeper pocketed professionals an advantage.
The WSOP quietly brought back rebuy events in 2016. For the Casino Employees event, which has no poker professionals, "players eliminated during registration period are allowed one re-entry".
The 2016 $111,111 High Roller for One Drop, $10,000 Deuce-to-Seven Draw Lowball Championship and $1,500 Deuce-to-Seven Draw Lowball each allowed one rebuy; while the $1,111 Little One for One Drop, $1,000 WSOP.com Online No-Limit Hold 'Em, and $565 Pot-Limit Omaha allowed unlimited rebuys.
An oddball 50/50 No-Limit Hold 'Em tournament in 2015 that paid half of its entrants is being ignored for this discussion. The event did not return for 2016.
Every year before 2012 had at least one event that paid under 10% of its field, and every year has had at least one event pay at least 13% of its field (with most years before 1989 going over 20%, a threshold that was also topped in 2012 and 2014).
The Main Event itself was winner-take-all through 1977, when 33 out of 34 players left empty-handed. The championship event paid between six and twelve percent from 1978 to 1985 then jumped to 26% in 1986, leading to seven straight years above 16%. This dropped almost steadily to 7% in 2001 and 2002 before creeping back up to 10% in 2005 where it remained until 2014. In 2015, since they thought it would be be a good selling point to pay the top 1,000 places, over 15% actually got paid.