finding dealer cost on used vehicles

I really doubt anyone at the DMV would have or give you such information.
The dealer is a reseller. He doesn't run down to the DMV with this information. And they don't ask for it that I know of. Plus, the transaction may be too current to be in anyones record.

Your best hope would be at the specific dealer itself. Either establish the right contact with somebody on the inside there or figure out the code they sometimes use on the sticker they place on the cars. See, the salesman needs to know the cost so he can figure out how to work you and sometimes they make it easy to read off the window sticker in case the salesman never passed the fourth grade or is just lazy.

The bottom line really is, WHY DO YOU CARE? If the dealer paid $1 for the car, would that mean he would sell it to you for $2, a mere 100% markup? Of course not. Dealers jiggle prices on vehicles all the time. The vehicle is worth as much as somebody will pay for it. Your job is to study enough offers and sales to know what a fair price is and work from their. And it is work if you want a good deal.

PS. You can check the "dealer trade" price on KBB and you will be astonished at how low the dealers take the vehicles in at. Buy low and sell high is every dealer's mantra. If you want a good deal, you need to buy from a private seller. You have to search out and drive to all the prospects. That takes time and work. That's why people are willing to pay more at a dealer. It just comes down to whether you are willing to exchange your effort and time for the money saved.