For my DS school awards banquet program, we put "full scholarship", no specific number, since we felt anything found on internet would be guessing. Wasn't comfortable putting down "equivalent to $xxx,000" type of thing, when USNA states tuition is zero. Glad you posted the question, maybe our approach was too conservative.
Per USNA Admissions page....."There is no tuition or financial aid at the Naval Academy. The Navy pays 100% of the tuition, room and board, medical and dental care costs of Naval Academy midshipmen. This means ALL students who attend the Naval Academy do so on a full scholarship in return for 5 years of active duty service upon graduation."