The Phillies introduced those mascots in 1971, and the Bicentennial was in 1976. Maybe the shirt has an MLB logo that is copyrighted 1968?
Anyway, it’s hard to put a price on it without listing it on eBay or in an auction. There is not a defined market price for it. But i bet you’d get at least $25 if it is unworn and in new shape. If beat up (or a kid size), don’t expect too much more than a few bucks.