I bought one a couple years ago for half the price you say on Amazon (Wired).
Keep in mind, these things are very loud, especially pressed up against a diaper (tons of vibration against the plastic), so if you have thin walls or live with a roommate I don't recommend getting one. Unless of course, you only use it when they aren't home…
But here is something not commonly talked about, vibrators tend to desensitize you real bad. If you like diaper humping, it won't do it for you and your general excitement dwindles the more you use it. It's fun to use once in a blue moon and quickly climax.
Personally, I recommend getting a small vibrator like the one Aneros Vice uses. I've found out that this little vibrator doesn't desensitize, but is also very pleasurable in a teasing sort of way, and is pretty quiet. It uses a single triple AAA battery and I've found that it tends to last for about 40 mins on a single battery… which is a lot of bang for the buck if you are getting cheapo Dollar Tree batteries that I use.
I tend to duct tape the vibrator onto of my diaper and hump it. Has three different vibration settings and many low/medium/high speeds. Unfortunately, however, it's about $120, but it also comes with a prostate massager. You stick the little vibrator into the Aneros Vice so you get vibrations against your G-Spot. Slides in easy with Astroglide gel.