An uncirculated (no wear or folds at all) bill with his signature can go on ebay to a collector for up to $5.
Please check your bill again. There are no 1968-date $1 bills. "Barr notes" are dated 1963.There's more information the at question "What is the value of a US 1 dollar bill signed by Secretary Joseph Barr?"
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If there are d $1 bills their value is $1 × d, or d dollars.Each $5 bills is worth 5 times as much so if there are f bills their value is $5 × f, or 5f dollars.Thus the expression for the total would be d + 5f dollars.
1 dollar bills
All 1963-series $1 bills, including the often-misunderstood "Barr notes", are common among collectors. As of 01/2010 they sell for about $1.25 if circulated, $2 to $2.50 uncirculated.
No US $1 bills are dated 1990.
The U.S. did not print any $1 bills with that date. In fact, NO U.S. bills have that date.
Internet frenzies to the contrary, "Barr dollars" are neither rare nor valuable.Please see "What is the value of a US 1 dollar bill signed by Joseph Barr?" for more information.
$1 bills dated 1974 and later are only worth face value.
Circulated U.S. $1 bills dated 1977 and later are only worth face value.
Please check your bills again and post a new, separate question. No US $1 bills were dated 1987.
25 two-dollar bills is equal in value to one fifty-dollar bill.