From THE #1 TRADE MAGAZINE FOR THE COIN-OPERATED ENTERTAINMENT INDUSTRY
Here's the raw data about how much people are making off as a pinball operator (~$44 a week) compared to darts and pool.
http://www.playmeter.com/images/SofIpg07.pdf
Funny thing... I just checked out a pin last weekend. Simpson party that earned over $30,000 in it's lifetime.