a Jameco ad from a September 1984 issue of BYTE magazine lists the 1MHz MC6809E at $14.95, compared with 2MHz MC6502A at $6.95, 4MHz Z80A at $4.49, and P8085A at $4.95. Same ad also lists 5MHz 8086 at $24.95

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The Atari ST was running a 68000 at 8 MHz. However, the 68000 needs four(!) cycles for each memory access - so the memory access time was in fact the same as my home-built 2 MHz 6502! But the Atari at least had a 16 bit data bus, so the total bandwidth was twice as much - on the other hand instructions took at least two bytes and were longer in general.
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