Irrelevant question really as it all depends on how fast a car sells doesn't it regarding cost per car , A car can be on 1 hour or 90 days , customers are the controlling force to that and lady luck sometimes
No it doesn't work on car values as to the percentage price to sell , if that was a case the site wouldn't attract the expensive stock .
So your answer is anything from a £1 as a nominal fee to load to a few £100 ,
This is not the secret to a site that performs well though , ITS MKT PENETRATION , the site HAS to get the cars under customers noses . Cant do that it WONT work . That's not easy or someone would compete with Autotrader
I see the expense of paying a site as a business decision , 1 ; charge me £200 a month and it doesnt work its a waste of money as it £2,400 a year down the drain ,2; charge me £2,000 a month and i sell 40 cars a month then its a better business proposition isn't it .