Put it this way.
$12,803,737 was Djokovic's prize money in 2012. Thats just prize money, not including any additional income from endorsements and deals etc.
$40,000 is approximately 0.3% of his yearly prize earnings.
Now put in in comparison. Lets assume the average person here earns around $40,000.
The equivalent impact to that persons earnings each year is $120.
Don't know about you guys, but I would pay $120 ANYDAY to have someone string and customise all my racquets.