Overvalued-
Old balloon tire cruisers. Nostalgia has pushed the price of these far beyond their actual value.
Low end bike boom 10 speeds, sure they don't cost a fortune, but anything over $20 for an old Murray or Free Spirit is too much.
Undervalued-
80's era Japanese and American road bikes. These were some really nice bikes and there are a lot of old Treks, Univegas, etc... selling for nearly the same prices as those old Murrays and Free Spirits. To a lot of people an old 10 speed is an old 10 speed.
Old mountain bikes. There were a lot of high end bikes made by the likes of Cannondale, Klein, Specialized, etc... that are now obsolete and very affordable.
Bridgestone seems to be the exception to both of the above. Nice bikes, but they're not usually cheap.