Determing Value of Automobile in Bankruptcy

Does anyone know if there is a particular guideline that trustees follow to determine the value of a car in bankruptcy? I could arguably come up with multiple values for the same automobile, depending whether I use the retail value, the wholesale value, or the auction value....and there may be other values to be considered. Since the means test uses IRS values for determining living expenses, one would think trustees would be required to use IRS methods for determining the value of assets like automobiles, but I don't think that is the case, but I am not sure about it. Does anyone know or have any experience with it?

I ask the question because I live in California and am looking at a chapter 7 bankruptcy. I do not owe any money on my car, which has a retail value of somewhere between $6,000.00 and $7,000.00. The wholesale value is about $4,000.00 and I imagine the auction value is somewhat lower. I believe the CA exemption is $2900.00. We want to keep the car, so I need to have a plan. I either need to borrow money, using the car for collateral, before the bankruptcy, or buy it from the trustee after the bankruptcy. I think it will be less expensive to borrow money, but I am not sure. There are many problems involved with either plan and I need to be careful to not tell too much to an attorney. If I borrow money, I have to spend it on something that the court will not take back (I am think of getting my teeth fixed in Tijuana). I am open to suggestions.