After the doves cry, there’s IRS Form 706.

Estate-tax attorneys for Prince, who died last week, must attempt to put a precise financial value on his name, image and likeness.

That Prince-ness could make him one of America’s top-earning deceased celebrities, and it may be one of his estate’s largest assets—subject to a 40% federal tax.

The Internal Revenue Service is used to putting price tags on tradeable assets and is well-trained in taking existing revenue streams and capitalizing them into a value. It is much trickier to divine the worth of a unique niche business—marketing Prince’s legacy—that doesn’t really exist yet.

There is no real precedent for Prince. The closest thing is the Michael Jackson estate-tax battle, headed for trial in the U.S. Tax Court in February.