If your credit score is bad, you are probably getting quoted a 15-25% interest rate (my first quoted interest rate was 25% at carmax when I first started working). They can recover depreciation pretty quickly in that case.

I'm assuming they don't give you a loan for the entire amount, i.e. just 70% of the value?

Throw in a prior car not paid off, paint/tire/oil protection and an additional warranty - it isn't hard to get up to or over 110% of the car's value.