Probably 2,000-2,500 baht if I would make a guess. In the case that you stayed home alone most of the time and cook/use the air-con during the day and night/do laundry.
How about transfer the kid to a gov hospital in BKK? At least gov. hospital in BKK would have better equipment and staffs. Not sure if it would be possible for a young child in critical condition to transfer for many hours though.
For my family, the groom present Sinsod as a proof that the couple have enough funds to proper raise a family before marriage. But for some other family, the parents will actually keep the Sidsod themselves.
I encourage you to start fill out the application form first and you will know what kind of documents you need to provide to the embassy. I think you’ll need an invitation letter and prove of sponsorship. If she travels a lot to other countries they will consider her a frequent traveler and it will help as well.