Miller was not represented in the Supreme Court, but cost was not the reason. Miller was a bootlegger arrested with a sawed-off shotgun. The trial judge held the National Firearms Act unconstitutional and let him go. When he heard the Government was going to appeal, he skipped to Canada.
I thought Miller was found dead after his case was decided (shot to death, with a loaded gun in his hand which had just fired some bullets other than what killed him). Perhaps he'd still have been alive if he'd had his trenchgun.