I was just curious, how often does the government actually prosecute cases of fraud in court?
Are there parameters that they usually follow when deciding to take a case to court?
"Fraud" is a broad term. In the first 9 months of Fiscal Year 2015, the US Government prosecuted 5173 white collar crimes, according to Syracuse University's Transactional Records Access Clearinghouse. This includes mail fraud, bank fraud, conspiracy to defraud the US, identity theft, health care fraud, wire fraud, and others (listed in the link).
Federal prosecutors have a large amount of discretion in when to bring charges. See The Department of Justice Policy on Charging and Sentencing and Title 9 of US Attorney's Manual, Section 27 (Principles of Federal Prosecution).
These are not laws, but internal department guidance to prosecutors that outline the factors they should consider when making charging decisions. There is quite a lot of detail in these guidelines, but at the highest level, they are summed up by this paragraph from the Attorney General's memo:
Accordingly, decisions regarding charging, plea agreements, and advocacy at sentencing must be made on the merits of each case, taking into account an individualized assessment of the defendant's conduct and criminal history and the circumstances relating to commission of the offense (including the impact of the crime on victims), the needs of the communities we serve, and federal resources and priorities.
The US Attorney's Manual gives very specific guidance regarding the prosecution of various forms of fraud. For example, Title 9, Section 43 covers mail and wire fraud. Here is an excerpt:
Prosecutions of fraud ordinarily should not be undertaken if the scheme employed consists of some isolated transactions between individuals, involving minor loss to the victims, in which case the parties should be left to settle their differences by civil or criminal litigation in the state courts.