What are the penalties of a US Citizen marrying an illegal alien for convenience to help the person get into the US and no longer wants to stay married?

There really is no risk for you. You are not going to lose your citizenship or go to jail. The risk is all for the person since they can get deported.

Penalties for participating could include perjury. Participation in a marriage fraud can result in up to five years in prison and/or up to $250,000 in fines per count. You take a huge risk. And just because you're not caught in the beginning doesn't mean you can't be later on a visit or stokes interview or whatnot.

However, if you decide to divorce, you can just like a regular marriage. Then it becomes the alien's job to file for non-joint application to remove conditional status. The judge will look at the marriage and try to decide if it was bona fide and that the divorce was for relational issues. If so, then he can allow the alien to get his green card, but that applies more towards the alien after the divorce. For the U.S. Citizen, you just divorce and if you're so inclined, sign one last piece of paper for him later. If he doesn't have it signed, he can't appeal to the 90 day period before the second anniversary and then he would be deported. But still, wouldn't affect the U.S. Citizen.