Are you talking about a standard residential style 120 VAC mini fridge?
Not sure if those use compressors... you might to check to see if they are based on a Peltier element because there are plenty of DC car fridges that use the same tech and can be had for about the same price.
My other concern is can it keep up with the increased ambient heat load... in car ambient temps can really get up there.
Being a mobile application, I would be concerned about energy efficiency and durability. Compressor based fridges have pretty amazing performance.
Peltier fridges might only cool about 40F below ambient. Its fine for drinks, but I don't really trust them in summer months to keep meat safe.
There are some budget DC fridges that are compressor based for not too much more money. ($300-400)
I have put my ARB compressor based fridge to the test (Vegas in the summer) and I was holding Freezer temps (13F) in 108F heat. I thought a 95 degree delta was pretty impressive. I think a budget fridge is fine, but you might have to manage your performance expectations.
I didn't want to spend $1000 on a fridge either, but my breaking point came after my budget option helped me get food poisoning when on a 3000 mile camping road trip. I spent about 1k renting a hotel room for 3 days until I recovered.