The LeMans started out as the top trim on the Pontiac Tempest in 1961. It would become its own model in the following year in 1962, and a convertible body style was added. It was also sold as a coupe.
The LeMans switched to the A platform in 1964. A sedan and station wagon body style was added, and the convertible was dropped. The top engine on these cars would be the 350 cubic inch V8, while the bottom engine was a 215 cubic inch inline 6 cylinder.
The LeMans was downgraded to a lower line in 1970. The engine was a huge 400 cubic inch V8, but the horsepower output was not great. A better 4 barrel carburetor version of that engine was a much higher performing option.
The LeMans got bigger and uglier throughout the 1970's. They were finally given a diet in 1978, becoming about 800 pounds lighter than the previous year's model and also getting downgraded to the midsize class. The V6 engine from Buick became standard, but V8 engines were still an option. An Oldsmobile diesel V8 was also sold with these cars before the LeMans was discontinued in 1981.
If you are looking to buy a Pontiac LeMans, you might have trouble finding a decent price guide to give you a value. I usually just check the listings on eBay and then figure out a price from there. Sometimes I just end up buying the car off eBay, because the prices tend to be so much cheaper than prices out in the "real world." There are some great deals on parts on there, too.