I am trying to find out how much money is spent on Pay Per Click
search engine advertising.

Overture's market share is 50%

http://www.michaelwong.com/archives/000095.html

Overture's revenue last year was $667 million.
http://www.sec.gov/Archives/edgar/data/1060439/000095014803000429/v88074e10vk.htm

I would guess its around $1.3 billion.

Impossible to answer, because Google never reveal their figures. It is
commonly thought that Google Adwords does the same amount of business
as Overture. Although there are hundreds of smaller PPC engines, they
would amount to much less that 10% of the market. I agree with
respree-ga, somewhere between 1 and 2 billion

Overture:

Overture reported first quarter 2003 revenue of $224.7 million, up 57
percent from revenue of $142.8 million in the first quarter of 2002.
http://www.corporate-ir.net/ireye/ir_site.zhtml?ticker=OVER&script=411&layout=0&item_id=404413

FindWhat (3rd biggest):

The Company announced that it believes revenue in Q2 2003 will be over
$17.0 million, versus Q1 2003 revenue of $15.8 million, and Q2 2002
revenue of $9.7 million.
http://www.findwhat.com/content/about/news/pressrelease.asp?a=113