You don't have to buy a Sausage McMuffin but you do have to ask for just a sausage patty as well as a hash brown. At my local McDonalds, the sausage alone goes for 89 cents, while a hash brown is $1.19 (yup, the meat costs less than the leftover potato trimmings).
My preferred approach is what I'm going to call "The Pork Surfer" (because it looks like a sausage on a potato surfboard). You just put the sausage on the hash brown and get the ideal pork-to-potato ratio. Behold!