I've flown with my girls quite a few times. The cost is outrageous but I've paid it most of the time...
After my first trip with Monroe I realized they don't really seem to check... When it was just Monroe she would be in a bag that looked like a purse and I realized I'd pay at the counter and then no one seemed to even notice whether or not the bag had a luggage tag (to test it I took the tag off once & no one said anything), the TSA didn't inquire (even when she came out of the bag to go through security) and the flight attendants never even knew she was there because she's so quiet... So, I did skip paying on the way back a couple times. I definitely felt guilty but I guess not guilty enough
Now that I have both of them I have paid the crazy fee; although I tested it once again by removing the luggage tag on the rolling carrier (which is pretty clearly a dog carrier) and kept the boarding pass with the "paid" stamp in my purse and used my original, and again had no one question it. If I were more of a risk taker I probably would have tried not paying again but with both of them I just feel like I need to to keep my sanity haha :-/