There can be a variety of reasons - usually due to the actual airline or popularity of your destination than where the airports are. For example, flights from London to New York, LA, Florida, Chicago and Vegas are usually much cheaper than flights from London to Little Rock, AR (which is where my aunt lives) - because hardly anyone flies from London to Arkansas compared to LA or New York.
Some airlines offer special advance prices or deals on seats, others sell the first seats at a cheap price and as the plane fills up, the price of a ticket increases. Prices may also vary depending on the day or time of day you are travelling - a word of advice, tickets are usually cheaper if you travel on a Monday to Thursday, because extra taxes are often added onto weekend flights (due to travelling businessmen/women and more people wanting to travel on the weekend).
It also depends on the time of year - for example, in February, a flight to Arkansas may be £300 (off peak season), but in July and August (peak season), tickets are £600-700.
Also, if you're looking at flights online, prices can vary from website to website. A ticket on Yahoo Travel may cost $600, but a ticket for exactly the same flight on Travelocity or Opodo may be much cheaper or more expensive.
Sometimes you can find a really cheap flight, but you have to make sacrifices in terms of seat comfort, leg room, meals offered and in-flight entertainment. Some UK airlines offer really cheap flights within Europe, but you have to pay extra for drinks, food and checked baggage, so at the end of the day, it doesn't work out that cheap at all!
I would advise looking around to find the best deal. Try Yahoo Travel, Travelocity, Opodo, Priceline, STA Travel etc. to see what prices are being offered and how well the flight times fit into your plans.
If you need any further help with understanding flight jargon or decisions on flights, doen't hesitate to ask
.