Dealer is often much cheaper than other sources and surprisingly cheap at that for more obscure parts like trim. It's the consumable items that seem to carry the excessive prices.
When I need something minor like this I search eBay for "mercedes *** Breaking" and email them for it. The ones that have individual listings for the items are 9 times out of 10 more expensive.
I've found they are actually quite rubbish at reading their own system so you need the number to be sure they are looking at the right bit for the right car.
You can find this for yourself by checking on one of the Russian hosted copies of EPS, search for a link on the forum as it's been posted several times. You'll need to enter your VIN to get it to load the right parts for your car, quite legit that just how it works. Google translate will do good enough job to find your way around and you can then work out exactly what you'll need.
As well as the main part number you'll also need the colour code which will be something like 7E94. When you have that you can check the price on Inchcape's online parts site which Google will also provide you with.