that the information you have posted does not confirm your assertion that people can only get one 1900 baht extension at immigration in a calendar year. In fact I have had 3 such 1900 baht extensions this calendar year and may apply for a 4th at the end of this month as opposed to crossing a border just after new year as I had planned. Please stop posting misinformation about this.
many people in Chiang Mai choose to extend at immigration as they don't have the time to spare to go up to Mae Sai ie 3 or 4 hours at immigration in Chiang Mai versus 12 hours, 8 of it on a bus there and back to Mae Sai. I gather when you take into account the cost of the bus fares or the fuel and possibly vehicle hire if you choose to drive and don't have your own vehicle, all adds to the 500 baht and for some people the saving just isn't worth it
I have heard of amounts of between 4 and 6 thousand Australian dollars to get the dog from the approved country back into Australia plus the costs of the dog staying for 6 months in an approved rabies free country assuming you knew someone in the approved country who would care for it for those 6 months