Prostitution was not illegal in the Soviet Union, but it was not legal either; the CPSU simply denied (up until the late 1980s) that there was prostitution in the country, and even then they said it was a byproduct of inviting Westerners to the country.
However, Moscow's police department had compiled a list of 3,500 prostitutes working in Moscow alone. Prostitutes were sometimes solicited by the KGB in order to collect information on foreign visitors, and successful prostitutes (those that could speak Western languages, etc.) could earn as much as 40,000 rubles (at that time, equal to $73,000 in 1987 dollars) annually.
Prostitutes that served Soviet citizens and foreign students often were paid far less, sometimes as little as $.40 for sexual services. All else I know is that Soviet prostitutes in the fifties were sometimes sent to hard labor camps, despite having committed no "crime" as designated by Soviet law.
Additional information is almost impossible to find on Soviet prostitution.