May 9, 2011: In the Netherlands, a former Dutch Air Force fighter pilot was recently arrested and charged with spying for Belarus. The former pilot was having financial problems, and was seeking to sell what he knew about the F-16s he used to fly. Belarus is a close ally of Russia, but also the last communist police state in Europe. Technically, Belarus is a democracy, but former communist era officials manipulated the voting to keep themselves in power, and use a Soviet era style security force to keep the opposition on the run. The Belorussian government is corrupt and the country is, not surprisingly, broke. So anything goes when it comes to raising cash. Illegal arms deals are a favorite. It appears that Belarus was seeking to get the F-16 data for resale, to the highest bidder.
With the end of the Cold War, more and more spies are doing it for the money. Actually, cash has long been one the main reasons people spy. During the Cold War, ideology was a big source of incentives for spies. Not so much anymore, although nationalism (a form of ideology) is frequently exploited by the Chinese, when turning ethnic Chinese living abroad into spies.
Conscience is still a good source of effective spies. People who are upset at the way their country is run, will often become spies, in hopes that they can bring down the current, hated, regime. During the Cold War, this was a common method for getting people in communist states (especially Russia) to become spies.
Compromise is an ancient technique for turning someone into a spy. It's basically blackmail. Anything will do. Honey pots (situations that end up producing photos of the subject having sex with someone they should not be screwing) is a longtime favorite. But any embarrassing secret will do.
Ego is often an incentive for people to spy. They do it because they are approached, and they find that the thrill of getting away with it, not the money, is the big draw. This is quite common. And ego-based spies, who might want to stop later, can be kept at it via blackmail (being turned in for spying.)
Spies often do it for a combination of reasons, but one of the five above is usually the main reason.