If you're going through inventory that quickly, $5 per car shouldn't be a problem.
Take this into account. Most New/Used dealerships have about 500-1000+ cars on hand and in inventory on a monthly basis. On average, dealers can sell 300 a month per location, easily. (at least in Chicago)
If you're making 300 new listings per month, that's $1,500 every single month for zero promises, zero obligations, just a simple listing on a website. No real way to track/measure performance. It's just as good as a news paper ad -
that's what craigslist was made to replace.
If you've got $1,500 to spend a month, you can make it go a much longer way by purchasing ad space and forming educated decisions, or having an SEO go through your website/webmastertools and remove/disavow cannibalistic links, which you'll see improve search metrics.
There's no tracking metric on craigslist, other than views, that'll help you improve your craigslist leadgen strategy. Not to mention.. it's a lot of work that can be better spent elsewhere.