HMRC recommended rate for business travel in own car is 45 pence per mile to include wear and tear, and probably a more fuel efficient car not towing a heavy object, so that is £225 just for that - if your car is older, the depreciation element might not be quite so much, but it's probably offset by more fuel and your time.
It depends how much you want to sell the item and how likely you are to find another buyer who will buy and collect. Also the price of the item to a degree. The buyer is far more likely to stomach a realistic delivery cost if the item is say £10k, rather than £200, unless you are selling something difficult to find.
I'd ask for at least £250, and let them take it or leave it, unless maybe you are desperate to get rid of the item, have no other buyers and like long journeys; I'd also consider whether it is safe/reasonable to drive that distance in one day. It could be if it's all motorway that's unlikely to be bad traffic, eg Sheffield to Ediburgh, but not if a lot of the 500 miles are slower and/or busier.
A commercial courier might also tow it, for a similar price, because they might be able to cover some or all the return journey with a similar job, especially if you are flexible with timescales.