Commission Structure
Amazon operates a volume-based advertising fee structure. The more products that are shipped as a result of your affiliate links, the more you'll make per sale. Once you have sold enough products to move up to a different advertising rate, all subsequent sales will give you commission at that rate, until and unless you reach the next fee level. Note that some products are exempt from this commission structure (see Fixed Fee Products below).
Fee Rates
For the first one to six sales, commission is paid at 4 percent of the product's price. At seven to 30 sales, commission increases to 6 percent. Between 31 and 110 sales, you'll earn 6.5 percent, between 111 and 320 sales you'll get 7 percent and between 321 and 630 you'll earn 7.5 percent. If you sell between 631 and 1570 products you'll get 8 percent commission, selling 1571 to 3130 products earns you 8.25 percent, and if you sell over 3131 products you'll make 8.5 percent per sale.
Fixed Fee Products
Some Amazon products are exempt from the stepped commission structure. These include cell phones and electronics (4 percent), video game downloads (10 percent), industrial products and anything on Amazonsupply.com (8 percent), Amazon gift cards (6 percent), third-party gift cards (4 percent), magazines (25 percent) and groceries (4 percent). Note that computers, including tablets and laptops, are capped at $25 commission while MP3 and instant video sales are capped at $1.50 maximum. All other products are eligible for the increasing advertising fees.
tl;dr...You need to sell more to get a higher cut. People who make decent money with Amazon definitely do volume and/or sell more expensive products.