Expanding upon what Fullblast has said, the term "low cost" is a bit like "short string" - all depends upon what you need.
Broadly speaking you can break costs down into:
- Purchase costs (with repayment spread over how long?)
- Operating crew costs
- Operating fuel / parts costs
- Operating maintenance personnel costs
The best thing then is to create some form of figure of merit equation based upon these factors (which you can probably get a reasonable idea of from Boeing/Airbus' etc. technical websites).
Then it really is a question of what you do with the aircraft. For very low utilisation then the equation falls in favour of the cheap, old and thirsty older airliner whilst on the other hand for high utilisation then a high value, low running cost aeroplanes makes much more sense. So you start to see why budget airlines are actually flying the newest and most expensive aeroplanes, but air taxi or smaller charter operators are often flying fairly old and expensive to run aeroplanes.
As for aircraft to look at - obviously "Janes all the world's aircraft" is the best starting source of reference. I'd take a look at the Boeing 737 and 767, Airbus A310, A320, A321, Antonov AN-76 for a starter.
G
N.B. If you plan on a career in the aircraft industry, stop using the word "plane" now. A plane is a wordworking tool, or a flat surface - not a flying machine. Also the plural of aircraft is aircraft, not aircrafts.