First of all, you need two pipes, a combustion inlet and an outlet vent. And they must be separated by at least four feet. Code also calls for a 6 inch diameter insulated outside connection to the cold air side of the furnace air and also an automatic damper on it. You must also deal with the acidic condensate. If you don't have a drain near the furnace, you may need a condensate reservor and pump. Then you also need some place to pump it.
I have all that. It wasn’t exactly cheap but not inordinately expensive
I have a 95% efficient natural gas furnace installed in a utility closet in the basement, and the installer used a horizontal concentric intake/outflow pipe design, one pipe inside another. It works fine and the intake/exhaust pipe is designed for the type furnace I have (not just something the installer threw together in other words).