When I was looking for my lease deal, which I did through First Call Leasing, I could have had the SLK for no more than the C220. Apparently a huge factor is the anticipated residual value at the end of the lease, the higher the RV the lower the cost.
As has already been said, they will tend to be lower spec, but having said that, I'm more than happy with the Executive SE spaec. I included Map Pilot, which cost me £5 a month, other extras or up-spec will add more.
Also, as VED is included, and mine is £0 year 1, £30 year 2, that affects the cost also, as of course does length of lease and mileage. The basic deals are 10,000 miles/year.
Don't forget that most won't include insurance or servicing, repairs will be covered by MB warranty.
(And I CHOSE a manual - the last RWD auto I had didn't like the wet or winter.........)