$50 for a MB oil change is not so bad. The filter will be around $8 and 8 liters of Shell Rotella is about $17 which totals $25 in parts. If the shop charges $100 per hour then this gives them 15 minutes for the oil change.
Don't let this analysis lead you to believe that I am taking my cars to anyone for an oil change. I am strictly DIY when it comes to me cars. No one cares about them like I do.