The standard guide is Eric Newman, The Early Paper Money of America, 4th ed. 1997.   In that work the retail values of the $8 note from the PA May 10, 1775 emission are as follows:  Good $20; Very Good $30; Fine $45; Very Fine $65; Extremely Fine $90; Uncirculated $180.  In general recent 1999 auction prices have been about 15%-20% higher than the Newman listings (which came out in January of 1998, but was copyrighted in 1997).

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Several later colocial era emissions including most Continental Congressand Pennsylvania emissions are in this general price range (with some exceptions).  Some other common emissions have even lower values (as MA 1780 and MD of 1770 and 1774) while other issues are rarer and command higher prices.  Rarity is more important than age, for example a one shilling note from the New Jersey emission of June 22, 1756 is valued at$174 in crisp uncirculated condition while a Massachusetts note of May 25,1775 is valued at $3,500 in very fine condition.