Locals were probably motivated broadly the same as any other foreign intelligence asset: political sympathy, a desire to even the geopolitical balance, money, deception, romance, blackmail. But it's also possible that some supplied civilian technological know-how in the hope of making the USSR more western or of boosting trade. A few in high-tech industries might even have hoped to profit from supplying key parts. It's a murky world.
Many of the major spies of the later Cold War period (that is, post-WWII, post-Stalin) were money and ego. Aldrich Ames and Robert Hanssen are the poster-boys for this combination, getting a lot of cash for their information, but also enjoying the superiority they felt about getting away with it (until, of course, they didn't). Money seems to have motivated many of the West German moles who were giving technical information (much lower-level than Ames or Hanssen) to the foreign branch of the Stasi as well. These were people who could rationalize it to themselves by pointing out (correctly) that the West wasn't always so high-and-mighty in turning its ideals into practice either, that in the post-Stalin period, the Soviet Union wasn't quite the obvious villain that it might have previously been, and that anyway, the supplying of one technical blueprint or another isn't going to change the balance of power all that much. (This is different from the kind of intelligence that Ames and Hanssen gave up, which led to a lot of deaths.)
Kristie Macrakis' Seduced by Secrets: Inside the Stasi's Spy-Tech World has some nice chapters on the recruitment of West German moles by the Stasi to pass on technical and industrial information in the later Cold War, the sort of thing that was the German analog to Line X. It was a very different sort of enterprise, and environment, than many of the more sensational spy stories (e.g. the atomic bomb, the wartime spies, even the really deep intel moles like Hanssen or Ames) — less about high-flying ideological sympathies and more about income augmentation mixed with getting a thrill out of being a spy.