$50 per click will never work, Carsales does about $50/click (for dealers) and in reality i'd look for a car on carsales and not on AE.
I couldn't see AE ads getting more than $5-$20 per car run it till you sell it for private ads as Carsales charges $60.
I would use it to bring traffic flow in. In the near future at least it would probably harm the website to charge too much, it would lead to people stop advertising, and then only the forum members remain talking about shit day it day out.
Dealers are a bit different, they understand that enquires = sales, i'm sure you could build the AE car sales side of things by aproaching dealers and giving them something crazy like $1 per enquiry, build the name up that way, hope they make a few sales and see how it goes.
But $50 is a no way scenario, the market leader charges that because they can, and in reality it is hard for any other website to take them on because the public now refers to used cars for sale as carsales.com.au
Like where the trading post 20 years ago was the beez kneez.
My 2 cents,