I did it a couple of months ago on my 63-plate C63. The old injectors all had a manufacture date of May 2013 stamped on them so they were original. I bought the injectors online and swapped them out myself so I only had to pay for the parts, about £256 from sparkplugs.co.uk.
Was that quote from a main dealer? Seems a bit on the steep side, any half-decent mechanic could probably do it in an hour, two tops. Think I took about three but it was my first job on this car really so still not that familiar with the engine bay.
But yes, I'd have it done or do it yourself if you can, peace of mind is worth a lot.