Consumers seem more and more unwilling to engage with "advertising" these days. Instead, they prefer "coming across" new companies and ideas in the belief they found something on their own and will then happily share it with their network. This challenges marketers to reach consumers with low-key approaches using the right channels at the right time. Where the B2B world can still find quality prospects with very focused and well-targeted messaging, in B2C, I believe it takes an omni-channel approach that lets consumers find you wherever they are most likely to look.
We worked with a client on a study of their Facebook advertising in a B2C market. Ads were getting good click-throughs, and we were able to measure the trend of clicks over a few weeks time. However, conversions were very low for this group... low enough that the client concluded Facebook ads were not working. But then we saw how organic traffic increased with the exact same trend during the same time. Conversions from this source were quite high. As this was the only advertising being run at the time, we concluded many consumers simply did not want to click on the ad they saw but instead visited the company's website directly. And, in so doing, perhaps felt like they had "discovered" the service on their own and signed up.
For your automative purchasing service, clearly hearing about the service from trusted networks will make the biggest difference, and all the social media channels are good for this. I think it's important to find incentives that be offered to the early adopters who are connectors and vocal. As I recall, Constant Contact found at one point that their average cost of customer acquisition was about $800 (in the very early days). They decided to offer this amount to current customers as a referral credit if they got a friend to sign up (I think the friend had to stay a customer for 10+ months). The program was a huge success, and in addition to staying within their budget, Constant Contact also developed their brand as trusted... something they could not have done on their own.
Consider an omni-channel approach for your automotive buying service and a strong referral program. Find ways to let consumers "find out" about you without pushing too much information directly at them. Email marketing, while it sounds good on the surface, may not be the best early approach for NEW customers, but it could be better suited for follow-up email to current customers for referrals. Retargeting makes a lot of sense with consumers as does advertising with partners. Of course you must track all channels from the get-go to understand which are resulting in conversions and which are not. It's important to be agile in your marketing with consumers, who change their patterns often. Only with proper conversion analysis can you truly understand where wasted ad spend results and opportunities exist.
In terms of actual numbers... I agree with Wayne that there are many variables to consider here. That said, I have worked with clients spending a few thousand dollars a month to get a couple hundred conversions each week up to $400k+/month for several thousand conversions.