Don't pay him much. It looks like some kind of Opuntia. A cutting would probably be easy to root in dry soil. Give it plenty of sun and not much water.
If you do get a piece don't handle it with your bare hands. Use tongs of some kind. It can have small spines that are very annoying if you get them in your skin.
Looks a lot like Opuntia cochenillifera to me, but I am not 100% sure. Should not be a problem to root from pads, but if you have to pay money make sure you get a few nice big fat ones, not one of the floppy thin ones, those will root too, but form a much less sturdy base.
I also think these might be a little frost tender - as it is a more (sub)tropical cactus. I got a box with about 20 pads on ebay from someone in Florida earlier this year, I forget what I paid, but it was not that much, less than a dollar/pad, I think.
Out of those 20 a few of the smaller (2-3 inches long) failed but the 3-4 really nice big fat pads rooted fine and a couple of them have shot up incredibly fast, the rest have rooted fine, but grown less fast. If the plant in the picture is indeed O. cochenillifera, it is supposed to be an arborescent Opuntia, so it can get really really big if given space. The pads are mostly thornless and apparently quite edible, it gets nice flowers and I think the fruits are edible too.
More appropriate for material like this is a trade of modest value: cookies, your own plant material, a glass of homemade lemonade... It doesn't look like I'd pay more than $.50 per pad, which makes a trade more fun and fitting.
I'm pretty sure it's Opuntia engelmannii var. linguiformis, commonly known as cow-tongue cactus. As mcvansoest said, it is easily propagated by simply by sticking a pad into the soil. This plant was originally found growing in the San Antonio, TX region but I've grown it in Fort Worth so I know that it's quite cold-tolerant. Around here, most of the native Opuntias are considered weeds. As far as the asking price of 5 dollars is concerned,I'll just say this. "One man' s weed is another man's treasure."
Looks more like the common O. ficus-indica that's been kept on the dry side and would possibly do better in the ground. The shape of the pads, the spacing of the areoles & the trunk style all point towards this. Comparison photos: http://davesgarden.com/guides/pf/showimage/316211/