If you are considering second-hand, then note that second-hand petrol C-Class are generally unloved/unwanted (unless C63.....) hence why the C180 will be much cheaper second-hand than any CDI of similar spec/trim/year.
The perception is that Diesel is more frugal, and the older the car the more likely that the potential buyer will have budget and running-costs in mind hence why petrol is generally shunned.
This may change if Diesel falls out of favour due to taxation/emissions etc, but for now second-hand petrol C-Class (from 180 to 250) offer great value compared to Diesels.