This is the pickle for many of us that don't drive monster SUVs. It's easy to make it work if you you're moving from something that glugs 17-20L/100km to 6-7. But, for those of us operating at 10L/100km or so, it's tough to justify a Yaris/Civic/etc.
My C230 gets 10L/10km in all-city driving and can actually get into the mid to low 6's on the highway. Sure, a Yaris is better, but since my C230 is now nearly nine years old, the price gap between my "old" Mercedes and the new car means I have to drive the Yaris until I retire to make the payback.
But, the Yaris is under warranty so I'd have lower maintenance/repair costs, right? Well, sure, but not by much. The Mercedes, despite it's 200K, is pretty much like new and takes little $$ to keep it that way. The depreciation on the Yaris is small, but still more than the MB at this stage in it's life.
So, after sitting down and really hammering it out, the Yaris (yeah, we were thinking about getting a new Yaris and selling the C230) just didn't make financial sense. It would have to be a "'cause I want it" decision, not because we'd actually save any money.
Now, my neighbour who drives his big ute literally from one end of the city to the other on his daily commute? I figured out he could KEEP the big ute and by leasing a new Yaris (and a well equipped one) the gas savings would MORE than make the lease payment and monthly insurance bill.