The situation may vary from dealer to dealer. Dealers in economic depressed areas are definitly not going to make their living selling new cars, that's for sure.
In general, I think the dealers makes a couple to a few thousand on the new cars they sell. Sometimes there are factory incentives that don't get passed down to the buyer. So, suppose they make 2 grand per car and within a 8 hour business day they sell 4 cars, that's 8 grand. On the service side, suppose on average they charge $80 for regular service and repairs per job/vehicle. How many cars would they have to service in a day to equal what they make by selling 4 cars?
Like I said... it's probably different from dealer to dealer. Some are service oriented (and have a lot of service bays)... some are more sales oriented. Depends on what their business model is.