It's still worth one dollar.
$10.
I believe the watermark began on the 1990 series so no 1988 was still too old.
There were no series letters on 1988 US $50 bills. You may be looking at the Federal Reserve District letter. Series letters, if present, are small and next to a bill's date. There's more information at the Related Question.
Sorry, there is no such bill.
It's still worth $50.
10.00 dollars
"A" is the only possible series letter for a 1988 $20 bill. You may be referring to the Federal Reserve District letter. The series letter, if any, on US bills is next to the date. In any case the bill is new enough that it's only worth face value if it's been in circulation. Uncirculated ones might be worth an extra dollar or two.
I would like to the current value of excellent condition of 1862 US series 268 1 dollar bill.
It is worth face value unless it is uncirculated.
Face value only.
Check
Face value only, unless it's crisp and unfolded.