Mitch, it's all about which keywords you select. What I basically do is this:
Monthly Search Volume x Click Thru Rate of Top 3 Listing x Conversion Rate x Closing Rate x Value of car deal = Value of keyword
So, let's say you find a keyword with 1100 searches a month:
1100 x 12% (rough estimate) x 5% x 15% x $2000 = $2000/month.
However, that doesn't account for walk in traffic. So I also do a rough estimate, based on historical trends from the dealer, usually around 2% of unique traffic converts to sales. This is just correlation, and not causation, but with the companies I've worked with I see this number stay about the same as we grow the visitor total with SEO.
This equation goes like this: Search Volume x click thru x conversion x Value of sale
So, that same 1100 monthly search keyword would look like this:
1100 x 12% x 2% x $2000 = $5200
From there I just create a range. So, I can estimate between a $2000-$5200/month from that keyword if I can get in the top 3.
As for content not being worth it, I would disagree. One of my websites was getting around 1000 visitors a month for used car searches, but the bounce rate was in the high 20% range. So, I created a landing page specifically for used cars, optimized it with internal links so that it ranked well, and now the majority of the traffic goes through that landing page with a bounce rate below 10%. I did the same thing for other searches, too. SEO isn't just about acquiring new visits, it's also about improving the experience of your current ones.
In the same sense as Yago, if you search "Toyota Tundra Crewmax 4x4 for sale" you will see this page:
2013 Toyota Tundra Crew Max 4x4 for Sale | Ressler Motors : Enjoy one of the most dependable trucks in its class: The new 2013 Toyota Tundra Crew Max 4x4
that page is not optimized well, and essentially just exists. However, it ranks very well for that phrase, and generates about a car sale a month. That being said, that specific dealer has partnered with a shipping company, and is able to get great rates on shipping cars across country. I saw a Facebook post by Subaru of Wichita saying that they offer free shipping in the U.S, so more dealers are doing it.