AFAIK, the feds won't get involved unless...

A.) It's a federal matter.. Ie. across state lines, or involves the federal government.

B.) The total money (damages, theft, whatever) is greater than at least $10,000.

I know that when a friend of mine got busted (I did not know him at the time, I met him afterward) the Secret Service made the bust, started the investigation, but dropped the case to the State of Florida when they found they only had about a 3-4,000 dollar case.