I&#x27;m using a PIC12 in product that replaces an existing one. At Volume Pricing 8 Bit PICs are pretty much incomparable to ARMs. A quick check on Digikey shows that for 10000 pieces, the cheapest 8 Bit Micro is a PIC10 - 35Cents&#x2F;Unit (that&#x27;s for today only). ATTiny is about 2 Cents more but that can change.  
The cheapest 8051 from Energy micro is in the same range as well.<p>The cheapest ARM Cortex M3&#x2F;M0 or even a ARM7 is $1+. When you are shipping products (and not doing a hobby project), BOM cost is a major factor. If you sell a Million Units each with a micro in them, the cost difference is about $700,000. Even keeping cost aside, size matters. The smallest 8 Bit micros from Atmel or Microchip or any 8051 is around 3x3mm. Most ARM micros are much larger.<p>Rule of thumb that I use is to pick the micro by function, price and size. Sometimes a 8-Bit micro is more than enough for the job. Buying something that costs more, bigger in size and for which the MHz is not an advantage is an overkill.<p>Having said that, a companion product that also working on requires the MHz and so, a Cortex-M3 fits perfects there.