I paid $6k for my car. When I bought it, I couldn't find secured financing because it was 7 years old and had over 110,000 miles on it. At the time, 7 years was generally the max age for vehicle financing and 100k was generally the max mileage. I financed it using my unsecured LOC at 9%, then immediately transferred the balance to a Chase Slate card (0%, 18 months, $0 transfer fee). I paid 4-5 days interest on the LOC, then paid it off before the 0% period on the credit card ended. Financing was DEFINITELY worth it.
If I were to buy a car today, it would be in the same general age/mileage range as the last one I bought. I think I could get secured financing this time since banks have relaxed their collateral requirements in the past few years... To determine whether I would want the loan I would add up the increased insurance cost, loan fee's, title fee's, etc., and balance that against average market returns for the same period of time. I would NOT pay a higher purchase price because of financing, or add warranty, or any of the other additional costs people sometimes get roped in to.