It's always tough to try to guess how popular particular processor families are in production products. It tends not to be advertised (no "AVR Inside" labels!), and in some cases is considered a trade secret.
(Once, when I was interested in PICs as a hobbyist, but working for a company that generally used large 32bit microprocessors, a Microchip salescritter called me up and offered to drop by my office for a visit. I did explain that my interest was mostly non-professional, but they came by anyway and told me things I hadn't known about just how many PICs our company used in "support" roles: temperature monitors, power supply sequencers, ... "Interesting.")
Be aware that "PIC" is highly ambiguous, encompassing 8bit PIC16 and (significantly different) PIC18 architectures, 16bit PIC24, dsPIC30 and dsPIC33C architectures, and 32bit PIC32 (MIPS), PIC32C (ARM), and dsPIC33A (new!) architectures.
Prices vary a lot, from less than $0.50 for small 8-bit PICs to about $30 for some of the larger dsPICs (digikey q1 prices.)
For a long time, the 8bit PICs were considered approximately comparable to the 8bit AVRs, and PICs were easier to get (as a hobbyist), but:
- AVRs became more easily available.
- They are less high-level-language friendly, and high-level programming became more "important."
- There are no (or few) high-quality, free, compilers (in particular, no GCC.)
- There is no C++ compiler.
- There is an architectural limit of 4k of RAM. (new AVRs have more than that.)
- Arduino doesn't run on 8bit PICs. (There is a core "ChipKit" for the PIC32)