Help finding appropriate AVR device
Digikey has excellent selection tools for its components. You can select on package, manufacturer, ADC, and other features. (Odd that memory size doen't seem to be important.)  

I found 143 parts matching your requirements. The ATmega48P is 1.76 dollar in 1s and has 4 kB of Flash. The ATmega88P and ATmega168P are the same, but with 8 kB and 16 kB Flash, resp (same datasheet).