Vintage pennants are difficult to date to a specific season. Many pennants were for sale at the ballparks for several years in a row. Team logo's and artwork could help in dating. Often teams will use the same artwork on other souvenirs like programs, and yearbooks making it possible to date to a specific year.
A full sized 1980 Philadelphia Phillies in general is worth about $10.-$15. in excellent to near/mint condition. As with all collectibles condition is important.
Common flaws with pennants are missing ties, pin holes, frying, creases, and "crackling" on the printing. Sometimes people will trim down a pennant to rid it of the frying or other flaws like a flat tip (point) Caused by being pinned up and ripped off. Any of these flaws will degrade the pennant significantly.