The car you want is just a year old. Typically, a car's value drops by 10% to 12% on its first year. I just configured a 2016 S4 with the same configuration as the CPO S4 you're eyeing. Brand new costs around $56,245. So, that $48k price tag for your CPO S4 sounds reasonable to me. However, I won't just take his asking price. Try to haggle for a lower price. Start with a $46,500 offer and work your way up to say $47,500 or so. Then, evaluate the outcome of the negotiation. Also, keep in mind not to act desperate or your haggling will be fruitless. A lot of sales people can easily sense whether you're desperate or not. If they see desperation, they'll use it so they won't budge on their asking price.
Another thing you should investigate is whether Audi has factory or dealer incentives for CPO cars. These incentives vary by area so do your research.
One other thing you can do to determine whether $48k price is reasonable is to look at similar used CPO S4's at a handful of other Audi dealers who allow you to search through their pre-owned inventory. For example, I went to an Audi dealer website in LA that has 1
CPO 2015 S4 with, more or less, the same option configuration and mileage as the one you want. Their asking price is $47,995. See what other dealers want for similar CPO cars. You'll have a better idea of the range of asking prices of CPO '15 S4's.
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