$2 bills have been issued since 1862 so it's not possible to give a specific answer without knowing the bill's date, series letter (if any) and seal color.
Assuming your bill is dated 1976 or later, though, it would be a standard Federal Reserve Note and would retail for maybe $3 to $4 if uncirculated. However the bills aren't rare. A dealer probably wouldn't offer much more than face value because they have all they need
In average condition it has no extra value. A crisp, uncirculated 1977 US $20 bill might retail for all of $22 or $23. A dealer will only pay face value.
As of 08/2013, its retail value would be only about $11 in average condition. A nice uncirculated one might sell for $17 or so.
"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.
Like all modern $1 bills, a 1999 US $1 bill is worth one dollar. If your question is asking if it's worth more than one dollar to a collector, these bills have no added value unless they're in the best possible uncirculated condition.
Please check your bill's date again. he US didn't print any bills of any denomination dated 1978. The nearest date for $2 bills is 1976; those bills are only worth face value unless they are uncirculated. Correction: Yes they did make a 1978 two dollar bill. I am looking at one and holding it in my hand.
As of 06/2014 it has no added value, even in uncirculated condition.
Unless it is uncirculated, any $20 bill printed since the 1970s is worth only face value.
A nice uncirculated one might retail for $3 but a dealer will only pay face value or maybe $2.25 if you're fortunate.
Face value only. A 2009 $1 bill is too common and too new to have any extra value, even in uncirculated condition.
The value of a 1999 US 1 dollar bill depends on its condition and rarity. In general, a circulated 1999 US 1 dollar bill is worth its face value of $1. However, if the bill is in uncirculated condition or has a unique printing error, it may be worth more to collectors. It is recommended to consult with a currency appraiser or collector for an accurate valuation.
Despite its age, in circulated condition it has no added value. Uncirculated, it might sell for all of $1.25
Please check your bill again and post a new, separate question. There are no US $1 bills dated 1997.
As of 03/2010 a 1974 US $10 bill retails for about $11 in circulated condition, and roughly $18 if crisp and uncirculated.
In average condition it has no extra value. A crisp, uncirculated 1977 US $20 bill might retail for all of $22 or $23. A dealer will only pay face value.
As of 11/2011, it might sell for $52 in nearly-uncirculated condition, $60 in crisp uncirculated condition. A dealer will pay face value only.
Unless it's crisp and uncirculated, it's only worth face value. If it's uncirculated and not folded, it could retail for as much as twice that amount.
These bills are not rare. In circulated condition they have no extra value. Even uncirculated, they retail for at most $1 more.