What is the value of a two cent Mark Hopkins stamp?

Answer 1

This stamp is not at all rare, but is over 60 years old, so mint copies are not

as common as they were and 2 cents was not the regular first class rate,

although unsealed cards went at that rate.

A dealer would charge from 10 to 20 cents for it mint or used.

The buying price from one large dealer is $1.50 per hundred used.

Unused in sheets you can probably get more than face.

Answer 2

Two pennies.

Or...

Currently (2011) anything from a few pence for a post-1854 perforated stamp in grotty condition, through a few quid for a 1841-1854 imperforate stamp, to £10s-£100s for a used 1840 imperforate stamp, to £1000s for a perfect mint 1840 issue stamp.

It depends on precise details of the stamp, its condition and the state of the market.