How do you calculate the value of a location for a billboard? 

Say you have a plot of land by a road, and you want to know how much
money you'd get / how much you'd be able to charge from people to
advertise on that space, if you put a billboard up - How would you go
about finding that out? Who are the people to ask or speak to who
would be able to calculate that information?

I'd drive down the road a bit to the nearest billboard, note the
company that sells the space, look them up, call and ask what their
rates are. You can take an average if you turn around after the first
billboard, drive the other way to the nearest billboard going in the
opposite direction...

Pricing is based on "impressions". In advertising lingo it's
effectively the number of people that see your message.

There is also an additional qualitative valuation based on the type of
billboard. Color, electronic, dynamic message billboards cost more
because the theory is that people will be attracted to them more than
static boards.

From the suppliers perspective, the price starts with material and
labor costs, gets a location cost tacked on (supplier may not own the
site). Any gain beyond their costs is margin associated with the site
(call it intangible value).

Cynthia is also correct, you can call the billboard provider and they
will give you a quote based on the type of billboard you want to place
(electronic vs static). In the business, this is called a rate card.
Parameters also include who does the graphics, how long the billboard
stays etc etc etc...

Pricing will vary by the traffic count in either direction as well.