Yep, Your just going to have to find a buyer you can trust to give you fair pricing.
There is a few companies on the net where they have pictures with their prices, might be able to print them off and put together your own flyer of cats. so you can glance at it and know.

From what I've heard, some buyers will have certain ones higher priced while others will be lower.
That's why I said, find one you trust to give you an overall good price.

Some of the guys on here have used this one with some luck.

Catalytic Converter Scrap List - Prices & Pictures

Pic's; Catalytic Converter ID: Federal Autocat:

Durham Core INC