If something is bargainous, it costs less than usual or less than you expected — it is a bargain, cheap in a good way. 'These shoes were bargainous.' It is informal British, a playful adjective; Americans almost never say it. They say a bargain, cheap, or a good deal instead. You would not put it in a contract, a textbook, or careful American English. Though actually, -ous usually means 'full of' or 'having that quality' (famous, dangerous), so bargainous is built like a joke: full of bargain. Learners sometimes treat it as a 'proper' formal adjective because of that ending. It is not. It is chatty British, and many careful speakers would just say cheap or a real bargain.