Replacing Exterior Back Door
It will often cost less to replace the door and frame as a unit. Custom-fitting a door is a fussy process. A pre-hung door and frame as a pre-matched set is a factory-built item. If you can get over the horror of ripping the frame out, it's generally faster to complete the job, as well.

However, if your doorframe is an architecturally significant old thing, replacing just the door may be the better approach - most new doors &amp; frames are best for replacing cookie-cutter ticky-tacky doors and frames as found on many, but not all houses; they will look just the same, in most cases. [pun intended - RIP, Mr. Seeger]