My dog was only in heat once when we first got her. She was in heat for a few weeks, from memory, but her blood loss was nothing like you have described, just a bit of spotting here and there.

I'd take your dog to the vet if I were you. That amount of blood loss sounds excessive. My understanding is the bleeding/spotting is a sign they are fertile, not like for humans when it is after our fertile period.

Our dog was de-sexed immediately after the bleeding stopped, when she was no longer on heat. She was in a bit of pain for a few days, although she tried to valiantly follow us around as soon as we brought her home! She had her op in the morning and we picked her up in the afternoon. The cost will vary from area to area, I'd check with your local vet. She had painkillers the day after, from memory, but not after that. The vet said she'd be more careful if she knew when it hurt, if that makes sense.