horses for courses. If you want a small microcontroller that has a very simple IO model then avr is great, and if you use something like an attiny they're cheap as ... chips.
At small scale the price difference is pretty meaningless or even non-existent. For example right now the cheapest AVR at quantity of 10 costs $0.837, while cheapest ARM is $0.714.
(prices checked from digikey)