It was cheaper because of volume - the die size was almost exactly the same - the chip was almost exactly the same. There was a 6500 before the 6502 (I had one) but it used a weird technology that meant it drew almost all its power from the clock lines (two phase non-overlapping clock) and the interface voltages were also non-standard, so the 6502 was magnificently better. The 6502 had two 8 bit pointers and could therefore do a string move or compare quite painlessly and any fool could write a Basic interpreter for it in a couple of weeks. After they designed the 6800, they realised that processing strings on a 6800 was hell's own job cos it only had one pointer (although it was 16 bit).
The 6800 was designed by the same people. The 6502 was years before the 6802 or Z80.