The main thing is that whatever you do, you need to have them on your side so if you get back to the studio and discover you need to do a second or third recording session, that (a) they will be happy to have you back again and (b) you haven't set such a high precedent that you can't afford to return!
It also depends how much of their time/resources I've used.... Most (non-professional) vehicle owners I've used are usually happy with $100 for a few hours of their & their vehicles time.... I often find vehicles to record by finding someone selling one, so to get an extra $100 for a vehicle they are selling can help them out, and usually people are interested in what you're doing & why.... Also think of their hourly rate, as you will be using their time... (If I ask them for a figure I always pay them more than they ask)
But if its a small thing or a favour then I agree with Iain, a nice bottle of red or bubbles or a dozen beer depending on their likely favourite, but appreciate some people dont drink and will appreciate something else (petrol voucher? chocolates?)
Being a scrapyard they are probably beer drinkers - did you notice any crates of empties? You definitely want to reward them as you may go back there again in the future....