A five dollar piss is that piss you take when you've been holding it for a longgggggggggggg time. Usually after over 3 beers. It is a piss that felt as good as finding $5 on the ground. In other words, the pleasure you get from finally letting it go would be worth $5 if you could somehow pay someone for a similar sensation.
Generally had when there isn't a good place to pee in public, and you have to wait until you are in a bathroom or approach a wooded area.
Coincides with saying "arghhhhhhhhhhhhhhhahhhhhhhhhhh" when you finally do get to urinate.
Damn dude, I've had to piss since we started walking across this huge parking lot. I'ma go take a five dollar piss.
The line for the bathroom was so long, I ended up taking a five dollar piss when I finally got to the bathroom.