the MPC8306 is a fairly high power microcontroller with Ethernet, USB, QUICC engine, etc so hence its price.
There are plenty of cheaper microcontrollers but the specification could well be lower, e.g. Microchip's PIC series
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You need to specify your requirements (program memory, RAM, EEPROM, IO devices, speed, etc) then look for devices which satisfy the specification.
I would tend to chose a more powerful device than the basic requirements to give expandability and flexibility - cutting down when producing thousands of units.