It's mostly based on what you paid, depending on what you paid. If you paid MSRP, then take the 35% off and another 18% the day you drive it off the lot.
It's the same with anything (although maybe not as steep.) If I buy a $30,000 car and drive it for one week and decide I don't like it, nobody is going to give me $30,000 for it since it would be better for them to go buy it from the dealer. It's a used car so unless you're a cash buyer you won't get as good of terms for financing so at the most it's worth maybe $25,000 as a one week old used car. Just the way the market works.