I had the same problem on my 1986 420SEL. The new air pumps are really expensive ($1200 CDN)and I was toying around with the idea of not replacing it. I found one at an auto recycler for $300 so I bought it and had the dealer put it in. They charged me $300 bucks for that. Apparently the mounting braket had rusted and they had to replace that too and it was hard to get out because it was siezed up, so it was still expensive, but not half what it could have been. Like Steve said, not an urgent fix, you can probably put it off for a while, but for the long term you might want to replace it if you can. If you have more urgent repairs to do use your money on them for now.
1995 E420 - retired
1986 420SEL - retired