The law pretty much speaks for itself on what happens to immigrants who commit marriage fraud -- that is, who enter into a sham marriage as a means of gaining U.S. permanent residence (a green card). The relevent law states that perpetrators can face prison, a fine, or both, as follows:
Any individual who knowingly enters into a marriage for the purpose of evading any provision of the immigration laws shall be imprisoned for not more than 5 years, or fined not more than $250,000, or both (I.N.A. § 275(c); 8 U.S.C. § 1325(c)).
The U.S. citizen or resident spouse could also face criminal prosecution, including fines or imprisonment, depending on the facts of the case. They are likely to be prosecuted for either criminal conspiracy (conspiring with the immigrant is enough; see U.S. v. Vickerage, 921 F.2d 143 (8th Cir. 1990)), or, if the facts support is, for establishing a “commercial enterprise” to get people green cards (see I.N.A. § 275(d); 8 U.S.C. § 1325(d)).
The extent to which these penalties are applied depends on the specifics of each case. The government tends to reserve the highest penalties for U.S. citizens or residents engaged in major conspiracy operations, such as systematically arranging fraudulent marriages. But that doesn’t mean that small-time participants in marriage fraud can count on a soft punishment. Ferreting out sham marriages is a top enforcement priority of the U.S. Department of Homeland Security (DHS). Even if the immigrants themselves are not prosecuted criminally, they will in all likelihood simply be deported (removed) and never allowed to return to the United States -- not even if they later enter into a real marriage with a U.S. citizen.