Yes, it is 15% for MOST categories. There's some categories that do 8% (like consumer electronics and cameras), and go up to 25% (kindle accessories).
It is tough to make a decent profit. We play the volume game. There are items that we may only make 50-80 cents on. But, we sell 200-300 of them in a month.
If you actually want to make good margins - stay away from the "best selling" items. Those typically have the lowest margins, and the most competition. Just like SEO, go to the medium / lower competition items when you are starting out. Your margins can be a lot higher, even though your volume won't be nearly as high.
The other option is to try and time the market on specific items when you believe there will be much more demand than supply. Playstation 4 / Xbox One would be a perfect example this holiday season.
Finally, the last thing you can do is source some sort of unique product that noone really has that makes sense for your niche. Order legit UPC codes from UPC code companies. Find best selling items on amazon, and 'bundle' your unique product with that best selling item, with a new UPC code. Why a new UPC code? Amazon requires you to list every item via UPC code. If you use the one on the best selling item, you'll just get attached to the existing listing, competing against 100's of sellers. If you "create an item", you can make a brand new listing with a new UPC code. Once you do this, do some offsite SEO to bump up the ranking of your listing. You'll likely get no competitors, because noone can recreate that unique product as part of the bundle.