What's the FMV of artwork that cannot be sold? IRS says $65 million

From the NY Times:

The object under discussion is "Canyon," a masterwork of 20th-century art created by Robert Rauschenberg that Mrs. Sonnabend’s children inherited when she died in 2007.
Because the work, a sculptural combine, includes a stuffed bald eagle, a bird under federal protection, the heirs would be committing a felony if they ever tried to sell it. So their appraisers have valued the work at zero.  But the Internal Revenue Service takes a different view. It has appraised “Canyon” at $65 million and is demanding that the owners pay $29.2 million in taxes.
At the moment, tax experts note that the I.R.S.’s stance puts the heirs in a bind: If they don’t pay, they would be guilty of violating federal tax laws, but if they try to sell “Canyon” to zero-out their bill, they could go to jail for violating eagle protection laws.
Mr. Lerner said that since the children assert the Rauschenberg has no dollar value for estate purposes, they could not claim a charitable deduction by donating “Canyon” to a museum. If the I.R.S. were to prevail in its $65 million valuation, he said the heirs would still have to pay the $40.9 million in taxes and penalties regardless of a donation.
Then, given their income and the limits on deductions, he said, they would be able to deduct only a small part of the work’s value each year. Mr. Lerner estimated that it would take about 75 years for them to absorb the deduction.
“So my clients would have to live to 140 or so,” he said.