My SL 550 is coming up on its first service B since I bought it. I am 82 and do not work on it. About how much would a service b cost at an independent Mercedes service specialist? What is normally done? The car has about 68000 miles on it and does not have any problems I am aware of.

same as A service with a few additions.

What is Service A?

With the first visit at approximately 10,000 miles or 1 year whichever comes first – and then approximately every 20,000 miles or 2 years after that - Service A includes:

• Mercedes-Benz motor oil replacement
• Oil filter replacement
• Fluid level checks and corrections
• Tire inflation check and correction
• Brake component inspection
• Reset maintenance counter

What is Service B?

With the first visit at approximately 20,000 miles or 1 year after the previous service, and then approximately every 20,000 miles or 2 years after that — Service B includes:

• Mercedes-Benz motor oil replacement
• Oil filter replacement
• Fluid level checks and corrections
• Tire inflation check and correction
• Cabin dust/combination filter replacement
• Brake component inspection
• Brake fluid exchange
• Reset maintenance counter

See here:
Mercedes Service & Maintenance | Mercedes-Benz USA