The 6502 was mainly designed by ex Motorola engineer Chuck Peddle and Bill Mensch. Peddle was involved in the design of the 6800, but was disappointed at Motorola when they refused to sell the chip cheaply enough to fit Peddle’s vision of a microprocessor in any device where it could be of use. Peddle left Motorola and joined MOS Technologies, then doing ASICs for calculators and started designing the new chip. The first version, 6501, was pin compatible with 6800, and Motorola sued MOS, resulting in the slightly different 6502.
While the 6800 was introduced at $300, the 6502 sold at $25. One of the early adopters at the Wescon show in September 1975 was Steve Wozniak, looking for a CPU for his homebrew computer, later known as the Apple I.
Further details can be found here, and in Brian Bagnall, “On the Edge: The Spectacular Rise and Fall of Commodore”, Variant Press, 2005
Computers in my collection based on 6502 and chips derived from it: