Nexium is a patent extension of Prilosec. In order to patent Nexium, they had to alter the chemical structure and run clinical trials to show that Nexium was an improvement over Prilosec. Some critics argued that the trials inflated the differences. IIRC, when some generic drug makers tried to sell generic versions of Nexium, AstraZeneca sued them and won.

Malcolm Gladwell, covered this and other issues affecting pharmaceutical pricing in a 2004 article. It's an even-handed article, worth reading the entire piece -

http://www.gladwell.com/2004/2004_10_25_a_drugs.html