Led,

I believe this is the answer to your question about salable "coupons." Text is at bottom.

Source:  http://www.fda.gov/ForIndustry/DevelopingProductsforRareDiseasesConditions/RarePediatricDiseasePriorityVoucherProgram/default.htm

You did not ask "how much they sell for," but behind the paywall at the WSJ lies the answer ($67MM USD) for at least one sale:

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=9&cad=rja&uact=8&ved=0ahUKEwiM-K_Y-5DSAhXH7IMKHcDWCTkQFghIMAg&url=http%3A%2F%2Fwww.wsj.com%2Farticles%2Fsanofi-regenerons-alirocumab-shows-promise-in-phase-iii-trials-1406738434&usg=AFQjCNHLUIUVnaoMGb6P1XeNoTbMQ7oUdw&sig2=QpkJw8q4_g6NA1Xh_7cazQ

Rare Pediatric Disease Priority Review Voucher Program

Under Section 529 to the Federal Food, Drug, and Cosmetic Act (FD&C Act), FDA will award priority review vouchers to sponsors of rare pediatric disease product applications that meet certain criteria. Under this program, a sponsor who receives an approval for a drug or biologic for a "rare pediatric disease" may qualify for a voucher that can be redeemed to receive a priority review of a subsequent marketing application for a different product.

On September 30, 2016, the Advancing Hope Act of 2016 (Public Law No: 114-229) amended Section 529 of the FD&C Act.  Among the changes, the term "rare pediatric disease" now means a disease that meets each of the following criteria:
       A.  The disease is a serious or life-threatening disease in which the serious or life-threatening manifestations primarily affect individuals aged from birth to 18 years, including age groups often called neonates, infants, children, and adolescents.
       B.  The disease is rare disease or conditions, within the meaning of Section 526.
The Act changed the language of Subsection (A) from, "The disease primarily affects individuals aged from birth to 18 years, including age groups often called neonates, infants, children, and adolescents."  The full text of the Advancing Hope Act is available at: https://www.gpo.gov/fdsys/pkg/BILLS-114s1878enr/pdf/BILLS-114s1878enr.pdf 

Effective 90 days after the enactment of the Advancing Hope Act of 2016, the sponsor of a rare pediatric disease product application that intends to request a priority review voucher must submit such request in a cover letter to their NDA/BLA submission.

On November 17, 2014, FDA issued a draft guidance titled Rare Pediatric Disease Priority Review Vouchers Draft Guidance for Industry.  Although this guidance does not reflect changes made to the law by the Advancing Hope Act of 2016, the guidance contains relevant information pertaining to the parts of the law not changed by the 2016 Act.  Specifically, this guidance explains how FDA plans to implement Section 529 of the FD&C Act, including the process by which sponsors who are interested in receiving rare pediatric disease priority review vouchers may first request designation of their drug or biological product ("drug") as a drug for a "rare pediatric disease".  While such designation is not required to receive a voucher, requesting this in advance will expedite a sponsor's future request for a priority review voucher.

Questions regarding designations under this program should be directed to OOPD at [email protected] or 301-796-8660.  Product-specific questions about rare pediatric disease priority review voucher eligibility should be directed to the appropriate review division within the Center for Drug Evaluation and Research (CDER) or the Center for Biologics Evaluation and Research (CBER).  General questions related to the rare pediatric disease priority review voucher program can be directed to OOPD at [email protected] or 301-796-8660 or to CDER's Rare Diseases Program at [email protected] or either 301-796-4842 or 301-796-4061.

When sponsors use a priority review voucher to obtain a priority review of a drug application, they are required to pay a fee in order to do so.  the Federal Register notice concerning the fee for redeeming a Rare Disease Priority Review Voucher in fiscal year 2017 program is available here: https://www.gpo.gov/fdsys/pkg/FR-2016-09-30/pdf/2016-23624.pdf

Section 529 of the FD&C Act is codified at 21 U.S.C. 360ff.  the full text is available at: https://www.gpo.gov/fdsys/pkg/USCODE-2012-title21/pdf/USCODE-2012-title21-chap9-subchapV-partB-sec360ff.pdf  Please not that this version does not contain the advancing Hope Act of 2016 amendments.