I got about a $50/mos difference between the C300 and CLA250 as well. Well worth the extra $. The C300 is head and shoulders above the CLA in terms of interior, driveline, and appearance. The CLA feels like MB phoned it in, the C300 is a very desirable car.
What kills the CLA is the numerous other cars that can be had for the same, or less money, that are much nicer. There's a ton of cars from Korean and Japanese makers as well as other Yuro brands.
Am I really noting that the Subaru STi has a nicer interior than a Mercedes? WTF? Someone pinch me, I must be having a nightmare.