I'm getting a job so I can save up for a new mercedes? How much is one that is a small car/convertible??

Are you talking new or used? A new Mercedes SLK convertible (the smallest model) has a price range of about $45,000 (SLK350) to $68,000 (SLK55 AMG).

you will be saving for a while they do not come cheep

You will be working and saving for a quite a while if you want a new one. If you want to consider a used one, even the first year of the SLK ( the smallest Benz convertible ), the 1998 model year, still commands over $10 000 for one in good shape.
You must also consider the insurance and maintenance costs of a Benz as well. They will be higher compared to many other vehicles.

I'd say about 30k, unless you're in the European market. If you end up gettting a used Mercedes remember the old adage ''there is no such thing as a cheap Mercedes.'' l have a 1983 300sd. It cost me 6k, and l love it to death, but even doing all of the work myself it costs an arm and a leg. Unless you're a proficient mechanic, my advice would be to wait until you're pulling down at least 50k a year as a single person household and then buy new with a warranty.