I recommend you stop wondering whether you should spend your extra 15$ on more beer or a sandwich, and just order an Ass Saver. You won't regret it, unless of course it doesn't fit your saddle, and then you will. These little emergency fenders are an awesome alternative to more cumbersome and expensive fenders.
Designed to keep your ass dry and free of water and mud, the Ass Saver does the trick. Since most people want to just have a bike, or go for what they can afford, instead of worrying whether or not your frame is going to be compatible for a fender set up, the Ass Saver is a great accessory to have and eliminates that problem.
It also weighs the same as a stack of pancakes and can fold under the seat, so you can forget it is there entirely. I'm sure if you live in the Pacific Northwest or are constantly riding through torrential downpours, the Ass Saver may not be your best option, but you have bigger problems anyway.