If we create an efficient mega-hydrogen producing plant,  what is the
end cost of a gallon of hydrogen and how much energy would the initial
source have to produce to power all the processes?

There are a few factors involved.  A clean source:  solar or wind. 
Pure water is needed.  Assume one cannot transport the pure water or
deplete the water from an inland source.  Assume one would have to
desalinate ocean water. One would then have to have a plant that
liquefies the hydrogen for transport.  The cost of transport is
assumed to be the same as the cost of transport of oil.

What is mega-hydrogen?

"What is mega-hydrogen?"

   I think the poster just means "a plant for producing LOTS of hydrogen".

The end cost and the initial energy required are used to determine the
efficiency of such a plant...so your question simplifies to "If we
create an efficient plant, how efficient is it?" which clearly cannot
be answered.

That's no need in creating a Hydrogen plant...
If you have got a Solar Panel this create elettric energy this energy
could be used to divide H2O into Hydrogen and Oxigen and so you can
have you hydrogen at home.... (it's work, same peaple just had this at
home)
(excuse me for my bad english)
So you don't need a big plant, everyone could be producew Hydrogen at
home and there is no cost in trasport hydrogen...
Hydrogen could also be created at service-station...

can't we just power our homes with patchouli?

Hydroelectric, solar and wind power can be used to generate
electricity, the electricity can be used to electrolyse water, it
doesn't need to be pure, so salt water will do, the major cost is
going to be transport, Hydrogen has to be under immense pressure to
keep it liquid at room temperature, cooling it, means it can be kept
at lower pressures, but we are talking hundreds below C. Carrying
Hydrogen about and storing at home is going to be too expencive till
stronger tanks or better storage methods are found. One is storing
hydrogen in a solution which when burnt/ reacted with releases the
energy. I feel that while hydrogen is an ultimate goal, alcohol or
ethanol created from sugar running in normal internal combustion
engines is a good intermediary measure with no Carbon cost to the
atmosphere. For diesel cars Sunflower/animal fats can be converted
into bio diesel.
The ethanol / biodiesel ideas burn stuff created from the atmosphere,
so while they were being created, CO2 was being taken from the
atmosphere, when they are burned it is simply put back.
www.howstuffworks.com is a good source for information about all these
topics.