Can be really very modest...
These are possibly the last cars ever built designed to be "maintained." There is a regular schedule of required maintainance, and that combined with 3,000 mile oil changes (diesels) can really keep the wolf away from the door.
Only really big expense in eight years with an '83 240D has been a rebuilt transmission ($1500). No computers to go up, no "timing belts" to be changed at 60k...just rock solid reliability for hundreds of thousands of miles. These cars were the last produced under the old "luxury" car philosophy where a high initial price was amortized over an amazing potential service life by the simple rituals of maintenance.