The 6809 was released in 1978, but looking at the usual source for price quotes for old computer components, Byte magazine, I cannot find any quotes for 1979. December 1980 lists it at $38, compared to its closest competitor, the Z80A, for $14.50.
Transistor counts for the Z80 and 6809 were 8500 and 9000 respectively, a very small difference. It sometimes happens that a new chip will be expensive for the first little while after release because the manufacturer is still debugging yield problems, but this was two full years later. Why was the 6809 still so much more expensive than the Z80?