The lack of transparency and dealer control is the reason why I got into starting my own boutique agency. I've been navigating the choppy waters of automotive SEM for the last 6 years and I know what dealerships hate, and I know how to avoid hitting the rocks.
Hidden markups are common headache large Vendors cause, and that pain is compounded when the Vendor lays out no PPC strategy before taking command of your dealership's digital marketing fate. So at the end of the day the dealer has a SEM provider taking ridiculous commissions (often 40%+ of total ad spend), no strategy (just some plug-and-play campaigns), and no transparency or ownership of your own data. Not to mention your account manager will have 50 or 60 other clients to attend to, but that's another story.
I avoid all this by charging a flat fee, and giving full access of all marketing platforms to the dealer. Also, I remove the middle man by having the dealer pay Google directly so every dollar they give actually goes to 100% advertising efforts.
This may seem like a thinly veil advertisement, but it's just what I've been hearing from GM's/IMM's for a half decade. I don't mean to play therapist but I can feel the stress in their voice when they discuss a poorly managed SEM account, this affects their life and those around them. It's personally motivating to me to dig someone out of a hole and it's why I'll keep doing this my way.