Cheating by using a computer to play chess for you is something that happens mostly on the middle levels of chess tournaments, and the reason for this is simple, a computer does not play like a human, one way in which online cheaters are caught is that their play is compared to the different chess engines and if their play matches one of those engines then we can know they are cheating, so at the highest level of play a GM will immediately recognize that his opponent is using a computer to play on his place, so such cheating is not something that can be sustained for the long term.
I have no idea how big cheating is on the chess world, but I would expect it to be very small in the higher levels. Once you reach IM or GM rank everybody knows each other. The people have been playing chess since their teens and played against each other in many tournaments. It's not like one guy just using cheat software for 2 weeks and becoming a new chess professional. That's why the ELO system is so important. I think that online only players with a long track record can play each other. The potential backlash from cheating, like ruining your career, being banned for life from chess tournaments and paying a big fine should prevent anyone professional to do such a think. For small online chess tournaments with no professionals I think that betting on it is no real option because of the cheating risk.