Several months ago we dropped ppc campaigns. I can detect no negative effects as a result. Website traffic has increased. Organic referrals have increased. I think we are one of the few dealerships that can get away with something like this. You have to look at the local data to see the reasons. Smaller market of 450,000 metro. Closest major metro is Dallas 3hrs away. Only one major same make competitor. Very recognizable name.Chevyland. I mean can you get any better?! I think ppc steals away A lot of what I would have gotten organically. Can you see this in google analytics? That's what I'm trying to find out.
Google Analytics would be a bad source for this. I'd look at your queries in your Google Webmaster tools. % of queries that clicked organic while PPC was running, and % that clicked when PPC was off. Webmaster only goes back 3 months, so you might have missed the boat on that. The reason why I would choose Webmaster Tools is because you would be comparing the rate at which people click through, and not clicks, which are dependent on so many other variables - weather, marketing, time of year.
A good test would be done with keywords with a high volume that you rank in the top 3 for. Since you aren't currently doing any PPC you could get your base line stats right now. Make sure to download your webmaster tools data each month so you don't lose it forever. Once you have a month or three of data you could turn your PPC back on for those particular terms. The budget probably won't be very high since it will be so controlled. You would then measure the impact on click through rates on your organic, as well as how much it cost to increase the click through on those terms overall.
Specifically, let's say you had a 30% click through rate organically for "used cars Dallas," which in pretend land gets 1000 searches a month. So your website visits are 300. Now, you turned on PPC, because you heard it was so great, and now your organic click through rate dropped to 28%. This brings your organic down to 280. However, your PPC ads did really well, and garnered an additional 7%, or 70 clicks. Your total just went up to 350.
Moving into the real world, if your website converts to a sale at 2%, including online, phone and walk-in, not doing PPC got you 6 cars. Doing PPC could have possibly gotten you 7. Was that additional $300 worth another car sale? Depends on your dealership, but there are quite a few that would take it.
Now these numbers are made up, obviously, and you are unlikely to get 7% click through on your PPC, but that's how I'd analyze it. How much traffic do you really gain from PPC when you dominate the search results all ready, and is it worth it?
Edited: I wanted to mention that I'm pro PPC when done correctly. There is a lot of data you can get from it that can help benefit your SEO.