That seems to be a very reasonable price though I warn you to expect about
14-15 mpg in the city with that trip mileage. I do about the same in my 420sel in a suburban setting and get that in summer. In the winter it plunges to 11-12 mpg because of the warmup cycle though I live in Canada. This might not be as big an issue where you live.
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I agree with everyone advice especially running the car through the car wash. Spread newspapers in the trunk covering the entire area. If there is a leak the spots will indcate where the leak is. If its under the trunk lid the rear window is leaking and you may have a rusting issue. This could be bad news. I've seen postings where people have paid up to $2500 for removing the window, installing a new gasket, and having body work done. If its small enough you might be able to fix by using a filler to fill the leak. I had a small leak in my rear window which I patched using window glazing putty. It's been 18 months and the leak hasn't reappeared.
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If its by the rear tailight the tailight gaskets are likely needing replacing or the fresh air exhaust flap needs to be looked at.
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If its somewhere along the lip of the trunk lid its likely a new trunk gasket will be needed.