Free: Contests & Raffles.
If you're truck camping on a cot, there's no reason to use an inflatable mattress. Like 2many said, big foam pad and done. Get it at Freddie's for like $20. If you're a real pansy like me, get 2.
You will like the memory foam and buddies will be jealous.
I know the subject is cot pads, but the type of cot also matters. Having sleep on military style cots a lot, like six months at a time, another six months, another, well you get the picture. After the military, the very best cot I have slept on are those Coleman's with little springs around them. They are the best with or without a pad. Note there are two sizes, I did know when I bought mine. My son bought the larger size and I would recommend most folks get it. Oh, and they come with pads.