
There is a size limit as to which dogs can travel in the cabin. They have to be in one of those holdalls and fit under the seat. An 8 to 9 week old Elkhound pup just about scrapes in, and two of my pups have flown in the cabin, one to Spain, and the other to America. Otherwise they have to go in the hold.
When I flew to Finland with Lexi outward the dog was very expensive excess baggage (charged at a higher rate than normal excess), but on internal flights there was just a flat fee of about £15 per trip, and the dog had to be in the hold in the flight container (varikennel). You did have to prebook, and no more than two dogs were allowed per flight.
Abroad it seems to be much cheaper and more common to fly your dogs, here in UK seems to be next to impossible, or prohibitively expensive..