Here are a couple of thoughts for you.
1) The provider does not need to be industry specific. It sound good on the surface, but not necessary. But they do have to be a compnay that's on top of their game. There are plenty of SEM / re-marketing "experts" out there and there it the problem. I sold into the dealerships years ago for one of the largest. They did do a good job, but you're right they can be pricey.
2) Pricing. Unlike traditional marketing, it's very easy to tie the Digital to ROI. Pricing should be a secondary concern. Example, if company A can get you a ROI of 4-1 and Company B gets you a ROI of 8-1 (hypothetical figures for sure) it's obvious who you would choose. But if you are basing it completely on price, you would have most likely went with A. That is a lot of missed opportunity!
The technology is available for a company that is not Automotive specific to do a great job. Maybe even a better job because they don't cater to hundreds or thousands of dealers.
If you've researched Mudd, and feel good, give them a try. Good company's will work on 30 contracts. (an option I could not offer with mega vendor) And why shouldn't they. If they are doing good for you, why would you leave. keep in mind you should see improvements in month 1. Ask questions, understand which analytics really matter. Ask how can they lower cost per click and at the same time increase conversion. They should be talking about targeted ad groups, specific landing pages and ongoing A/B testing.....
Hope that helps